6. 벡터 레이어 사용

힌트

여러분이 PyQGIS 콘솔을 사용하지 않는 경우 이 페이지에 있는 코드 조각들을 다음과 같이 가져와야 합니다:

from qgis.core import (
  QgsApplication,
  QgsDataSourceUri,
  QgsCategorizedSymbolRenderer,
  QgsClassificationRange,
  QgsPointXY,
  QgsProject,
  QgsExpression,
  QgsField,
  QgsFields,
  QgsFeature,
  QgsFeatureRequest,
  QgsFeatureRenderer,
  QgsGeometry,
  QgsGraduatedSymbolRenderer,
  QgsMarkerSymbol,
  QgsMessageLog,
  QgsRectangle,
  QgsRendererCategory,
  QgsRendererRange,
  QgsSymbol,
  QgsVectorDataProvider,
  QgsVectorLayer,
  QgsVectorFileWriter,
  QgsWkbTypes,
  QgsSpatialIndex,
  QgsVectorLayerUtils
)

from qgis.core.additions.edit import edit

from qgis.PyQt.QtGui import (
    QColor,
)

이 장에서는 벡터 레이어에 대해 할 수 있는 여러 가지 작업들을 소개합니다.

Most work here is based on the methods of the QgsVectorLayer class.

6.1. Retrieving information about attributes

You can retrieve information about the fields associated with a vector layer by calling fields() on a QgsVectorLayer object:

vlayer = QgsVectorLayer("testdata/airports.shp", "airports", "ogr")
for field in vlayer.fields():
    print(field.name(), field.typeName())

ID Integer64
fk_region Integer64
ELEV Real
NAME String
USE String

The displayField() and mapTipTemplate() methods of the QgsVectorLayer class provide information on the field and template used in the 표시 속성 tab.

When you load a vector layer, a field is always chosen by QGIS as the Display Name, while the HTML Map Tip is empty by default. With these methods you can easily get both:

vlayer = QgsVectorLayer("testdata/airports.shp", "airports", "ogr")
print(vlayer.displayField())

NAME

참고

If you change the Display Name from a field to an expression, you have to use displayExpression() instead of displayField().

6.2. 벡터 레이어 상의 반복 작업

Iterating over the features in a vector layer is one of the most common tasks. Below is an example of the simple basic code to perform this task and showing some information about each feature. The layer variable is assumed to have a QgsVectorLayer object.

# "layer" is a QgsVectorLayer instance
layer = iface.activeLayer()
features = layer.getFeatures()

for feature in features:
    # retrieve every feature with its geometry and attributes
    print("Feature ID: ", feature.id())
    # fetch geometry
    # show some information about the feature geometry
    geom = feature.geometry()
    geomSingleType = QgsWkbTypes.isSingleType(geom.wkbType())
    if geom.type() == QgsWkbTypes.PointGeometry:
        # the geometry type can be of single or multi type
        if geomSingleType:
            x = geom.asPoint()
            print("Point: ", x)
        else:
            x = geom.asMultiPoint()
            print("MultiPoint: ", x)
    elif geom.type() == QgsWkbTypes.LineGeometry:
        if geomSingleType:
            x = geom.asPolyline()
            print("Line: ", x, "length: ", geom.length())
        else:
            x = geom.asMultiPolyline()
            print("MultiLine: ", x, "length: ", geom.length())
    elif geom.type() == QgsWkbTypes.PolygonGeometry:
        if geomSingleType:
            x = geom.asPolygon()
            print("Polygon: ", x, "Area: ", geom.area())
        else:
            x = geom.asMultiPolygon()
            print("MultiPolygon: ", x, "Area: ", geom.area())
    else:
        print("Unknown or invalid geometry")
    # fetch attributes
    attrs = feature.attributes()
    # attrs is a list. It contains all the attribute values of this feature
    print(attrs)
    # for this test only print the first feature
    break

Feature ID:  1
Point:  <QgsPointXY: POINT(7 45)>
[1, 'First feature']

6.3. 피처 선택

In QGIS desktop, features can be selected in different ways: the user can click on a feature, draw a rectangle on the map canvas or use an expression filter. Selected features are normally highlighted in a different color (default is yellow) to draw user’s attention on the selection.

Sometimes it can be useful to programmatically select features or to change the default color.

To select all the features, the selectAll() method can be used:

# Get the active layer (must be a vector layer)
layer = iface.activeLayer()
layer.selectAll()

To select using an expression, use the selectByExpression() method:

# Assumes that the active layer is points.shp file from the QGIS test suite
# (Class (string) and Heading (number) are attributes in points.shp)
layer = iface.activeLayer()
layer.selectByExpression('"Class"=\'B52\' and "Heading" > 10 and "Heading" <70', QgsVectorLayer.SetSelection)

To change the selection color you can use setSelectionColor() method of QgsMapCanvas as shown in the following example:

iface.mapCanvas().setSelectionColor( QColor("red") )

To add features to the selected features list for a given layer, you can call select() passing to it the list of features IDs:

selected_fid = []

# Get the first feature id from the layer
feature = next(layer.getFeatures())
if feature:
    selected_fid.append(feature.id())

# Add that features to the selected list
layer.select(selected_fid)

To clear the selection:

layer.removeSelection()

6.3.1. 속성에 접근하기

Attributes can be referred to by their name:

print(feature['name'])

First feature

Alternatively, attributes can be referred to by index. This is a bit faster than using the name. For example, to get the second attribute:

print(feature[1])

First feature

6.3.2. 선택된 피처에 대한 반복 작업

If you only need selected features, you can use the selectedFeatures() method from the vector layer:

selection = layer.selectedFeatures()
for feature in selection:
    # do whatever you need with the feature
    pass

6.3.3. 피처 부분 집합에 대한 반복 작업

If you want to iterate over a given subset of features in a layer, such as those within a given area, you have to add a QgsFeatureRequest object to the getFeatures() call. Here’s an example:

areaOfInterest = QgsRectangle(450290,400520, 450750,400780)

request = QgsFeatureRequest().setFilterRect(areaOfInterest)

for feature in layer.getFeatures(request):
    # do whatever you need with the feature
    pass

For the sake of speed, the intersection is often done only using feature’s bounding box. There is however a flag ExactIntersect that makes sure that only intersecting features will be returned:

request = QgsFeatureRequest().setFilterRect(areaOfInterest) \
                             .setFlags(QgsFeatureRequest.ExactIntersect)

With setLimit() you can limit the number of requested features. Here’s an example:

request = QgsFeatureRequest()
request.setLimit(2)
for feature in layer.getFeatures(request):
    print(feature)

<qgis._core.QgsFeature object at 0x7f9b78590948>
<qgis._core.QgsFeature object at 0x7faef5881670>

If you need an attribute-based filter instead (or in addition) of a spatial one like shown in the examples above, you can build a QgsExpression object and pass it to the QgsFeatureRequest constructor. Here’s an example:

# The expression will filter the features where the field "location_name"
# contains the word "Lake" (case insensitive)
exp = QgsExpression('location_name ILIKE \'%Lake%\'')
request = QgsFeatureRequest(exp)

See 표현식을 이용한 속성값의 필터링 및 계산 for the details about the syntax supported by QgsExpression.

request를 각 피처에서 검색된 데이터를 정의하는 데 사용할 수 있기에, 반복 작업자가 모든 피처를 순회하긴 하지만, 각 피처에 대한 부분적인 데이터를 반환합니다.

# Only return selected fields to increase the "speed" of the request
request.setSubsetOfAttributes([0,2])

# More user friendly version
request.setSubsetOfAttributes(['name','id'],layer.fields())

# Don't return geometry objects to increase the "speed" of the request
request.setFlags(QgsFeatureRequest.NoGeometry)

# Fetch only the feature with id 45
request.setFilterFid(45)

# The options may be chained
request.setFilterRect(areaOfInterest).setFlags(QgsFeatureRequest.NoGeometry).setFilterFid(45).setSubsetOfAttributes([0,2])

6.4. 벡터 레이어 수정

Most vector data providers support editing of layer data. Sometimes they support just a subset of possible editing actions. Use the capabilities() function to find out what set of functionality is supported.

caps = layer.dataProvider().capabilities()
# Check if a particular capability is supported:
if caps & QgsVectorDataProvider.DeleteFeatures:
    print('The layer supports DeleteFeatures')

The layer supports DeleteFeatures

For a list of all available capabilities, please refer to the API Documentation of QgsVectorDataProvider.

To print layer’s capabilities textual description in a comma separated list you can use capabilitiesString() as in the following example:

caps_string = layer.dataProvider().capabilitiesString()
# Print:
# 'Add Features, Delete Features, Change Attribute Values, Add Attributes,
# Delete Attributes, Rename Attributes, Fast Access to Features at ID,
# Presimplify Geometries, Presimplify Geometries with Validity Check,
# Transactions, Curved Geometries'

벡터 레이어 편집을 위한 다음 방법 가운데 어떤 것을 사용하든, 변경 사항은 기저 데이터 저장소(파일, 데이터베이스 등등)에 직접 반영됩니다. 일시적으로만 변경하고자 할 경우, 편집 버퍼로 수정 하는 방법을 설명하는 다음 단계로 넘어가십시오.

참고

If you are working inside QGIS (either from the console or from a plugin), it might be necessary to force a redraw of the map canvas in order to see the changes you’ve done to the geometry, to the style or to the attributes:

# If caching is enabled, a simple canvas refresh might not be sufficient
# to trigger a redraw and you must clear the cached image for the layer
if iface.mapCanvas().isCachingEnabled():
    layer.triggerRepaint()
else:
    iface.mapCanvas().refresh()

6.4.1. 피처 추가

Create some QgsFeature instances and pass a list of them to provider’s addFeatures() method. It will return two values: result (True or False) and list of added features (their ID is set by the data store).

To set up the attributes of the feature, you can either initialize the feature passing a QgsFields object (you can obtain that from the fields() method of the vector layer) or call initAttributes() passing the number of fields you want to be added.

if caps & QgsVectorDataProvider.AddFeatures:
    feat = QgsFeature(layer.fields())
    feat.setAttributes([0, 'hello'])
    # Or set a single attribute by key or by index:
    feat.setAttribute('name', 'hello')
    feat.setAttribute(0, 'hello')
    feat.setGeometry(QgsGeometry.fromPointXY(QgsPointXY(123, 456)))
    (res, outFeats) = layer.dataProvider().addFeatures([feat])

6.4.2. 피처 삭제

To delete some features, just provide a list of their feature IDs.

if caps & QgsVectorDataProvider.DeleteFeatures:
    res = layer.dataProvider().deleteFeatures([5, 10])

6.4.3. 피처 수정

It is possible to either change feature’s geometry or to change some attributes. The following example first changes values of attributes with index 0 and 1, then it changes the feature’s geometry.

fid = 100   # ID of the feature we will modify

if caps & QgsVectorDataProvider.ChangeAttributeValues:
    attrs = { 0 : "hello", 1 : 123 }
    layer.dataProvider().changeAttributeValues({ fid : attrs })

if caps & QgsVectorDataProvider.ChangeGeometries:
    geom = QgsGeometry.fromPointXY(QgsPointXY(111,222))
    layer.dataProvider().changeGeometryValues({ fid : geom })

팁

Favor QgsVectorLayerEditUtils class for geometry-only edits

If you only need to change geometries, you might consider using the QgsVectorLayerEditUtils which provides some useful methods to edit geometries (translate, insert or move vertex, etc.).

6.4.4. 편집 버퍼로 벡터 레이어 수정

When editing vectors within QGIS application, you have to first start editing mode for a particular layer, then do some modifications and finally commit (or rollback) the changes. All the changes you make are not written until you commit them — they stay in layer’s in-memory editing buffer. It is possible to use this functionality also programmatically — it is just another method for vector layer editing that complements the direct usage of data providers. Use this option when providing some GUI tools for vector layer editing, since this will allow user to decide whether to commit/rollback and allows the usage of undo/redo. When changes are committed, all changes from the editing buffer are saved to data provider.

The methods are similar to the ones we have seen in the provider, but they are called on the QgsVectorLayer object instead.

For these methods to work, the layer must be in editing mode. To start the editing mode, use the startEditing() method. To stop editing, use the commitChanges() or rollBack() methods. The first one will commit all your changes to the data source, while the second one will discard them and will not modify the data source at all.

To find out whether a layer is in editing mode, use the isEditable() method.

Here you have some examples that demonstrate how to use these editing methods.

from qgis.PyQt.QtCore import QVariant

feat1 = feat2 = QgsFeature(layer.fields())
fid = 99
feat1.setId(fid)

# add two features (QgsFeature instances)
layer.addFeatures([feat1,feat2])
# delete a feature with specified ID
layer.deleteFeature(fid)

# set new geometry (QgsGeometry instance) for a feature
geometry = QgsGeometry.fromWkt("POINT(7 45)")
layer.changeGeometry(fid, geometry)
# update an attribute with given field index (int) to a given value
fieldIndex =1
value ='My new name'
layer.changeAttributeValue(fid, fieldIndex, value)

# add new field
layer.addAttribute(QgsField("mytext", QVariant.String))
# remove a field
layer.deleteAttribute(fieldIndex)

In order to make undo/redo work properly, the above mentioned calls have to be wrapped into undo commands. (If you do not care about undo/redo and want to have the changes stored immediately, then you will have easier work by editing with data provider.)

Here is how you can use the undo functionality:

layer.beginEditCommand("Feature triangulation")

# ... call layer's editing methods ...

if problem_occurred:
  layer.destroyEditCommand()
  # ... tell the user that there was a problem
  # and return

# ... more editing ...

layer.endEditCommand()

The beginEditCommand() method will create an internal “active” command and will record subsequent changes in vector layer. With the call to endEditCommand() the command is pushed onto the undo stack and the user will be able to undo/redo it from GUI. In case something went wrong while doing the changes, the destroyEditCommand() method will remove the command and rollback all changes done while this command was active.

You can also use the with edit(layer)-statement to wrap commit and rollback into a more semantic code block as shown in the example below:

with edit(layer):
  feat = next(layer.getFeatures())
  feat[0] = 5
  layer.updateFeature(feat)

This will automatically call commitChanges() in the end. If any exception occurs, it will rollBack() all the changes. In case a problem is encountered within commitChanges() (when the method returns False) a QgsEditError exception will be raised.

6.4.5. 항목 추가 및 제거

필드(속성)를 추가하려면, 필드를 정의한 리스트를 지정해야 합니다. 필드를 삭제하려면 필드 인덱스 목록만 넘겨주면 됩니다.

from qgis.PyQt.QtCore import QVariant

if caps & QgsVectorDataProvider.AddAttributes:
    res = layer.dataProvider().addAttributes(
        [QgsField("mytext", QVariant.String),
        QgsField("myint", QVariant.Int)])

if caps & QgsVectorDataProvider.DeleteAttributes:
    res = layer.dataProvider().deleteAttributes([0])

# Alternate methods for removing fields
# first create temporary fields to be removed (f1-3)
layer.dataProvider().addAttributes([QgsField("f1",QVariant.Int),QgsField("f2",QVariant.Int),QgsField("f3",QVariant.Int)])
layer.updateFields()
count=layer.fields().count() # count of layer fields
ind_list=list((count-3, count-2)) # create list

# remove a single field with an index
layer.dataProvider().deleteAttributes([count-1])

# remove multiple fields with a list of indices
layer.dataProvider().deleteAttributes(ind_list)

데이터 제공자에서 필드를 추가 또는 제거한 다음 레이어의 필드를 업데이트해야 합니다. 이는 변경 사항이 자동적으로 반영되지 않기 때문입니다.

layer.updateFields()

팁

Directly save changes using with based command

Using with edit(layer): the changes will be committed automatically calling commitChanges() at the end. If any exception occurs, it will rollBack() all the changes. See 편집 버퍼로 벡터 레이어 수정.

6.5. 공간 인덱스 사용

벡터 레이어에 대해 자주 쿼리를 해야 할 경우 공간 인덱스를 사용하면 코드 실행 속도를 획기적으로 향상시킬 수 있습니다. 여러분이 보간 알고리듬을 작성하는데, 보간값을 계산하기 위해 주어진 위치에서 가장 가까운 포인트 레이어의 포인트 10개를 알아내야 한다고 한번 상상해보십시오. 공간 인덱스가 없다면, QGIS가 그 포인트 10개를 찾는 유일한 방법은 해당 위치에서 모든 포인트까지의 거리를 각각 계산한 다음 그 거리들을 비교하는 것입니다. 특히 이 작업을 몇 군데의 위치에 대해 반복해야 할 경우 시간이 아주 오래 걸릴 수 있습니다. 레이어가 공간 인덱스를 가지고 있다면, 훨씬 효율적으로 작업할 수 있습니다.

공간 인덱스가 없는 레이어를 전화번호가 정렬되지도 색인되지도 않은 전화번호부라고 생각해보십시오. 어떤 사람의 전화번호를 찾으려면 처음부터 그 번호를 찾을 때까지 읽을 수밖에 없습니다.

Spatial indexes are not created by default for a QGIS vector layer, but you can create them easily. This is what you have to do:

• create spatial index using the QgsSpatialIndex class:

  index = QgsSpatialIndex()
  

• add features to index — index takes QgsFeature object and adds it to the internal data structure. You can create the object manually or use one from a previous call to the provider’s getFeatures() method.

  index.addFeature(feat)
  

• alternatively, you can load all features of a layer at once using bulk loading

  index = QgsSpatialIndex(layer.getFeatures())
  

• 공간 인덱스가 채워지면 쿼리를 해보십시오.

  # returns array of feature IDs of five nearest features
  nearest = index.nearestNeighbor(QgsPointXY(25.4, 12.7), 5)
  
  # returns array of IDs of features which intersect the rectangle
  intersect = index.intersects(QgsRectangle(22.5, 15.3, 23.1, 17.2))
  

You can also use the QgsSpatialIndexKDBush spatial index. This index is similar to the standard QgsSpatialIndex but:

• supports only single point features

• is static (no additional features can be added to the index after the construction)

• is much faster!

• allows direct retrieval of the original feature’s points, without requiring additional feature requests

• supports true distance based searches, i.e. return all points within a radius from a search point

6.6. The QgsVectorLayerUtils class

The QgsVectorLayerUtils class contains some very useful methods that you can use with vector layers.

For example the createFeature() method prepares a QgsFeature to be added to a vector layer keeping all the eventual constraints and default values of each field:

vlayer = QgsVectorLayer("testdata/airports.shp", "airports", "ogr")
feat = QgsVectorLayerUtils.createFeature(vlayer)

The getValues() method allows you to quickly get the values of a field or expression:

vlayer = QgsVectorLayer("testdata/airports.shp", "airports", "ogr")
# select only the first feature to make the output shorter
vlayer.selectByIds([1])
val = QgsVectorLayerUtils.getValues(vlayer, "NAME", selectedOnly=True)
print(val)

(['AMBLER'], True)

6.7. Creating Vector Layers

There are several ways to generate a vector layer dataset:

• the QgsVectorFileWriter class: A convenient class for writing vector files to disk, using either a static call to writeAsVectorFormatV3() which saves the whole vector layer or creating an instance of the class and issue calls to addFeature(). This class supports all the vector formats that GDAL supports (GeoPackage, Shapefile, GeoJSON, KML and others).

• the QgsVectorLayer class: instantiates a data provider that interprets the supplied path (url) of the data source to connect to and access the data. It can be used to create temporary, memory-based layers (memory) and connect to GDAL vector datasets (ogr), databases (postgres, spatialite, mysql, mssql) and more (wfs, gpx, delimitedtext…).

6.7.1. From an instance of QgsVectorFileWriter

# SaveVectorOptions contains many settings for the writer process
save_options = QgsVectorFileWriter.SaveVectorOptions()
transform_context = QgsProject.instance().transformContext()
# Write to a GeoPackage (default)
error = QgsVectorFileWriter.writeAsVectorFormatV3(layer,
                                                  "testdata/my_new_file.gpkg",
                                                  transform_context,
                                                  save_options)
if error[0] == QgsVectorFileWriter.NoError:
    print("success!")
else:
  print(error)

# Write to an ESRI Shapefile format dataset using UTF-8 text encoding
save_options = QgsVectorFileWriter.SaveVectorOptions()
save_options.driverName = "ESRI Shapefile"
save_options.fileEncoding = "UTF-8"
transform_context = QgsProject.instance().transformContext()
error = QgsVectorFileWriter.writeAsVectorFormatV3(layer,
                                                  "testdata/my_new_shapefile",
                                                  transform_context,
                                                  save_options)
if error[0] == QgsVectorFileWriter.NoError:
    print("success again!")
else:
  print(error)

# Write to an ESRI GDB file
save_options = QgsVectorFileWriter.SaveVectorOptions()
save_options.driverName = "FileGDB"
# if no geometry
save_options.overrideGeometryType = QgsWkbTypes.Unknown
save_options.actionOnExistingFile = QgsVectorFileWriter.CreateOrOverwriteLayer
save_options.layerName = 'my_new_layer_name'
transform_context = QgsProject.instance().transformContext()
gdb_path = "testdata/my_example.gdb"
error = QgsVectorFileWriter.writeAsVectorFormatV3(layer,
                                                gdb_path,
                                                transform_context,
                                                save_options)
if error[0] == QgsVectorFileWriter.NoError:
  print("success!")
else:
  print(error)

You can also convert fields to make them compatible with different formats by using the FieldValueConverter. For example, to convert array variable types (e.g. in Postgres) to a text type, you can do the following:

LIST_FIELD_NAME = 'xxxx'

class ESRIValueConverter(QgsVectorFileWriter.FieldValueConverter):

  def __init__(self, layer, list_field):
    QgsVectorFileWriter.FieldValueConverter.__init__(self)
    self.layer = layer
    self.list_field_idx = self.layer.fields().indexFromName(list_field)

  def convert(self, fieldIdxInLayer, value):
    if fieldIdxInLayer == self.list_field_idx:
      return QgsListFieldFormatter().representValue(layer=vlayer,
                                                    fieldIndex=self.list_field_idx,
                                                    config={},
                                                    cache=None,
                                                    value=value)
    else:
      return value

  def fieldDefinition(self, field):
    idx = self.layer.fields().indexFromName(field.name())
    if idx == self.list_field_idx:
      return QgsField(LIST_FIELD_NAME, QVariant.String)
    else:
      return self.layer.fields()[idx]

converter = ESRIValueConverter(vlayer, LIST_FIELD_NAME)
opts = QgsVectorFileWriter.SaveVectorOptions()
opts.fieldValueConverter = converter

A destination CRS may also be specified — if a valid instance of QgsCoordinateReferenceSystem is passed as the fourth parameter, the layer is transformed to that CRS.

For valid driver names please call the supportedFiltersAndFormats() method or consult the supported formats by OGR — you should pass the value in the “Code” column as the driver name.

Optionally you can set whether to export only selected features, pass further driver-specific options for creation or tell the writer not to create attributes… There are a number of other (optional) parameters; see the QgsVectorFileWriter documentation for details.

6.7.2. Directly from features

from qgis.PyQt.QtCore import QVariant

# define fields for feature attributes. A QgsFields object is needed
fields = QgsFields()
fields.append(QgsField("first", QVariant.Int))
fields.append(QgsField("second", QVariant.String))

""" create an instance of vector file writer, which will create the vector file.
Arguments:
1. path to new file (will fail if exists already)
2. field map
3. geometry type - from WKBTYPE enum
4. layer's spatial reference (instance of
   QgsCoordinateReferenceSystem)
5. coordinate transform context
6. save options (driver name for the output file, encoding etc.)
"""

crs = QgsProject.instance().crs()
transform_context = QgsProject.instance().transformContext()
save_options = QgsVectorFileWriter.SaveVectorOptions()
save_options.driverName = "ESRI Shapefile"
save_options.fileEncoding = "UTF-8"

writer = QgsVectorFileWriter.create(
  "testdata/my_new_shapefile.shp",
  fields,
  QgsWkbTypes.Point,
  crs,
  transform_context,
  save_options
)

if writer.hasError() != QgsVectorFileWriter.NoError:
    print("Error when creating shapefile: ",  writer.errorMessage())

# add a feature
fet = QgsFeature()

fet.setGeometry(QgsGeometry.fromPointXY(QgsPointXY(10,10)))
fet.setAttributes([1, "text"])
writer.addFeature(fet)

# delete the writer to flush features to disk
del writer

6.7.3. From an instance of QgsVectorLayer

Among all the data providers supported by the QgsVectorLayer class, let’s focus on the memory-based layers. Memory provider is intended to be used mainly by plugin or 3rd party app developers. It does not store data on disk, allowing developers to use it as a fast backend for some temporary layers.

이 제공자는 문자열(string), 정수(int), 더블형 실수(double) 유형의 필드를 지원합니다.

The memory provider also supports spatial indexing, which is enabled by calling the provider’s createSpatialIndex() function. Once the spatial index is created you will be able to iterate over features within smaller regions faster (since it’s not necessary to traverse all the features, only those in specified rectangle).

A memory provider is created by passing "memory" as the provider string to the QgsVectorLayer constructor.

The constructor also takes a URI defining the geometry type of the layer, one of: "Point", "LineString", "Polygon", "MultiPoint", "MultiLineString", "MultiPolygon" or "None".

이 URI로 좌표계, 항목, 메모리기반 제공자의 인덱스 작업도 설정할 수 있습니다. 문법은 다음과 같습니다.

crs=definition

Specifies the coordinate reference system, where definition may be any of the forms accepted by QgsCoordinateReferenceSystem.createFromString()

index=yes

제공자가 공간 인덱스를 사용하도록 설정합니다.

field=name:type(length,precision)

레이어의 속성을 설정합니다. 속성의 명칭은 필수적이며, 선택적으로 유형(정수, 더블형 실수, 문자 스트링), 길이 및 정밀도를 설정할 수 있습니다. 여러 개의 속성 항목을 정의할 수도 있습니다.

다음은 이 모든 옵션들을 포함하는 URI의 예시입니다.

"Point?crs=epsg:4326&field=id:integer&field=name:string(20)&index=yes"

다음은 메모리기반 제공자를 생성하고 값을 채우는 코드의 예시입니다.

from qgis.PyQt.QtCore import QVariant

# create layer
vl = QgsVectorLayer("Point", "temporary_points", "memory")
pr = vl.dataProvider()

# add fields
pr.addAttributes([QgsField("name", QVariant.String),
                    QgsField("age",  QVariant.Int),
                    QgsField("size", QVariant.Double)])
vl.updateFields() # tell the vector layer to fetch changes from the provider

# add a feature
fet = QgsFeature()
fet.setGeometry(QgsGeometry.fromPointXY(QgsPointXY(10,10)))
fet.setAttributes(["Johny", 2, 0.3])
pr.addFeatures([fet])

# update layer's extent when new features have been added
# because change of extent in provider is not propagated to the layer
vl.updateExtents()

마지막으로, 모든 작업이 성공적이었는지 확인해봅시다.

# show some stats
print("fields:", len(pr.fields()))
print("features:", pr.featureCount())
e = vl.extent()
print("extent:", e.xMinimum(), e.yMinimum(), e.xMaximum(), e.yMaximum())

# iterate over features
features = vl.getFeatures()
for fet in features:
    print("F:", fet.id(), fet.attributes(), fet.geometry().asPoint())

fields: 3
features: 1
extent: 10.0 10.0 10.0 10.0
F: 1 ['Johny', 2, 0.3] <QgsPointXY: POINT(10 10)>

6.8. 벡터 레이어의 표현(심볼)

벡터 레이어를 렌더링할 때, 레이어와 관련된 렌더러 와 심볼 이 데이터의 표현을 결정합니다. 심볼은 피처의 시각적 표현을 그리는 일을 담당하는 클래스이며, 렌더러는 특정 피처에 대해 어떤 심볼을 적용할지 결정합니다.

The renderer for a given layer can be obtained as shown below:

renderer = layer.renderer()

그리고 이를 참고해 조금 더 나가봅시다.

print("Type:", renderer.type())

Type: singleSymbol

There are several known renderer types available in the QGIS core library:

유형	클래스	설명
singleSymbol	QgsSingleSymbolRenderer	모든 피처를 동일한 심볼로 렌더링합니다.
categorizedSymbol	QgsCategorizedSymbolRenderer	피처를 각 카테고리에 대해 서로 다른 심볼로 렌더링합니다.
graduatedSymbol	QgsGraduatedSymbolRenderer	피처를 값의 범위에 따라 서로 다른 심볼로 렌더링합니다.

There might be also some custom renderer types, so never make an assumption there are just these types. You can query the application’s QgsRendererRegistry to find out currently available renderers:

print(QgsApplication.rendererRegistry().renderersList())

['nullSymbol', 'singleSymbol', 'categorizedSymbol', 'graduatedSymbol', 'RuleRenderer', 'pointDisplacement', 'pointCluster', 'mergedFeatureRenderer', 'invertedPolygonRenderer', 'heatmapRenderer', '25dRenderer', 'embeddedSymbol']

텍스트 형식으로 렌더러의 내용을 덤프받을 수도 있습니다. 디버깅 작업에 유용할 수 있습니다.

renderer.dump()

SINGLE: MARKER SYMBOL (1 layers) color 190,207,80,255

6.8.1. 단일 심볼 렌더러

You can get the symbol used for rendering by calling symbol() method and change it with setSymbol() method (note for C++ devs: the renderer takes ownership of the symbol.)

You can change the symbol used by a particular vector layer by calling setSymbol() passing an instance of the appropriate symbol instance. Symbols for point, line and polygon layers can be created by calling the createSimple() function of the corresponding classes QgsMarkerSymbol, QgsLineSymbol and QgsFillSymbol.

The dictionary passed to createSimple() sets the style properties of the symbol.

For example you can replace the symbol used by a particular point layer by calling setSymbol() passing an instance of a QgsMarkerSymbol, as in the following code example:

symbol = QgsMarkerSymbol.createSimple({'name': 'square', 'color': 'red'})
layer.renderer().setSymbol(symbol)
# show the change
layer.triggerRepaint()

name 은 마커의 형태를 의미하며, 다음 중 어느 것이라도 가능합니다.

• circle

• square

• cross

• rectangle

• diamond

• pentagon

• triangle

• equilateral_triangle

• star

• regular_star

• arrow

• filled_arrowhead

• x

To get the full list of properties for the first symbol layer of a symbol instance you can follow the example code:

print(layer.renderer().symbol().symbolLayers()[0].properties())

{'angle': '0', 'cap_style': 'square', 'color': '255,0,0,255', 'horizontal_anchor_point': '1', 'joinstyle': 'bevel', 'name': 'square', 'offset': '0,0', 'offset_map_unit_scale': '3x:0,0,0,0,0,0', 'offset_unit': 'MM', 'outline_color': '35,35,35,255', 'outline_style': 'solid', 'outline_width': '0', 'outline_width_map_unit_scale': '3x:0,0,0,0,0,0', 'outline_width_unit': 'MM', 'scale_method': 'diameter', 'size': '2', 'size_map_unit_scale': '3x:0,0,0,0,0,0', 'size_unit': 'MM', 'vertical_anchor_point': '1'}

This can be useful if you want to alter some properties:

# You can alter a single property...
layer.renderer().symbol().symbolLayer(0).setSize(3)
# ... but not all properties are accessible from methods,
# you can also replace the symbol completely:
props = layer.renderer().symbol().symbolLayer(0).properties()
props['color'] = 'yellow'
props['name'] = 'square'
layer.renderer().setSymbol(QgsMarkerSymbol.createSimple(props))
# show the changes
layer.triggerRepaint()

6.8.2. 카테고리 심볼 렌더러

When using a categorized renderer, you can query and set the attribute that is used for classification: use the classAttribute() and setClassAttribute() methods.

카테고리 목록을 얻는 방법은 다음과 같습니다.

categorized_renderer = QgsCategorizedSymbolRenderer()
# Add a few categories
cat1 = QgsRendererCategory('1', QgsMarkerSymbol(), 'category 1')
cat2 = QgsRendererCategory('2', QgsMarkerSymbol(), 'category 2')
categorized_renderer.addCategory(cat1)
categorized_renderer.addCategory(cat2)

for cat in categorized_renderer.categories():
    print("{}: {} :: {}".format(cat.value(), cat.label(), cat.symbol()))

1: category 1 :: <qgis._core.QgsMarkerSymbol object at 0x7f378ffcd9d8>
2: category 2 :: <qgis._core.QgsMarkerSymbol object at 0x7f378ffcd9d8>

Where value() is the value used for discrimination between categories, label() is a text used for category description and symbol() method returns the assigned symbol.

The renderer usually stores also original symbol and color ramp which were used for the classification: sourceColorRamp() and sourceSymbol() methods.

6.8.3. 등급 심볼 렌더러

이 렌더러는 앞에서 설명한 카테고리 심볼 렌더러와 매우 비슷하지만, 범주 당 하나의 속성을 할당하는 대신 값의 범위에 따라 할당하기 때문에 숫자 속성에만 사용할 수 있습니다.

이 렌더러가 사용하는 범위에 대해 상세히 조회하려면 다음과 같은 방법을 사용합니다.

graduated_renderer = QgsGraduatedSymbolRenderer()
# Add a few categories
graduated_renderer.addClassRange(QgsRendererRange(QgsClassificationRange('class 0-100', 0, 100), QgsMarkerSymbol()))
graduated_renderer.addClassRange(QgsRendererRange(QgsClassificationRange('class 101-200', 101, 200), QgsMarkerSymbol()))

for ran in graduated_renderer.ranges():
    print("{} - {}: {} {}".format(
        ran.lowerValue(),
        ran.upperValue(),
        ran.label(),
        ran.symbol()
      ))

0.0 - 100.0: class 0-100 <qgis._core.QgsMarkerSymbol object at 0x7f8bad281b88>
101.0 - 200.0: class 101-200 <qgis._core.QgsMarkerSymbol object at 0x7f8bad281b88>

you can again use the classAttribute() (to find the classification attribute name), sourceSymbol() and sourceColorRamp() methods. Additionally there is the mode() method which determines how the ranges were created: using equal intervals, quantiles or some other method.

사용자 정의 등급 심볼 렌더러를 생성하고자 할 경우 다음 예시에서처럼 하면 됩니다. (이 예시에서는 범주 2개인 간단한 배열을 생성합니다.)

from qgis.PyQt import QtGui

myVectorLayer = QgsVectorLayer("testdata/airports.shp", "airports", "ogr")
myTargetField = 'ELEV'
myRangeList = []
myOpacity = 1
# Make our first symbol and range...
myMin = 0.0
myMax = 50.0
myLabel = 'Group 1'
myColour = QtGui.QColor('#ffee00')
mySymbol1 = QgsSymbol.defaultSymbol(myVectorLayer.geometryType())
mySymbol1.setColor(myColour)
mySymbol1.setOpacity(myOpacity)
myRange1 = QgsRendererRange(myMin, myMax, mySymbol1, myLabel)
myRangeList.append(myRange1)
#now make another symbol and range...
myMin = 50.1
myMax = 100
myLabel = 'Group 2'
myColour = QtGui.QColor('#00eeff')
mySymbol2 = QgsSymbol.defaultSymbol(
     myVectorLayer.geometryType())
mySymbol2.setColor(myColour)
mySymbol2.setOpacity(myOpacity)
myRange2 = QgsRendererRange(myMin, myMax, mySymbol2, myLabel)
myRangeList.append(myRange2)
myRenderer = QgsGraduatedSymbolRenderer('', myRangeList)
myClassificationMethod = QgsApplication.classificationMethodRegistry().method("EqualInterval")
myRenderer.setClassificationMethod(myClassificationMethod)
myRenderer.setClassAttribute(myTargetField)

myVectorLayer.setRenderer(myRenderer)

6.8.4. 심볼 다루기

For representation of symbols, there is QgsSymbol base class with three derived classes:

• QgsMarkerSymbol — for point features

• QgsLineSymbol — for line features

• QgsFillSymbol — for polygon features

Every symbol consists of one or more symbol layers (classes derived from QgsSymbolLayer). The symbol layers do the actual rendering, the symbol class itself serves only as a container for the symbol layers.

Having an instance of a symbol (e.g. from a renderer), it is possible to explore it: the type() method says whether it is a marker, line or fill symbol. There is a dump() method which returns a brief description of the symbol. To get a list of symbol layers:

marker_symbol = QgsMarkerSymbol()
for i in range(marker_symbol.symbolLayerCount()):
    lyr = marker_symbol.symbolLayer(i)
    print("{}: {}".format(i, lyr.layerType()))

0: SimpleMarker

To find out symbol’s color use color() method and setColor() to change its color. With marker symbols additionally you can query for the symbol size and rotation with the size() and angle() methods. For line symbols the width() method returns the line width.

기본적으로 크기 및 두께는 밀리미터 단위이며, 각도는 도 단위입니다.

6.8.4.1. 심볼 레이어 다루기

As said before, symbol layers (subclasses of QgsSymbolLayer) determine the appearance of the features. There are several basic symbol layer classes for general use. It is possible to implement new symbol layer types and thus arbitrarily customize how features will be rendered. The layerType() method uniquely identifies the symbol layer class — the basic and default ones are SimpleMarker, SimpleLine and SimpleFill symbol layers types.

You can get a complete list of the types of symbol layers you can create for a given symbol layer class with the following code:

from qgis.core import QgsSymbolLayerRegistry
myRegistry = QgsApplication.symbolLayerRegistry()
myMetadata = myRegistry.symbolLayerMetadata("SimpleFill")
for item in myRegistry.symbolLayersForType(QgsSymbol.Marker):
    print(item)

AnimatedMarker
EllipseMarker
FilledMarker
FontMarker
GeometryGenerator
MaskMarker
RasterMarker
SimpleMarker
SvgMarker
VectorField

The QgsSymbolLayerRegistry class manages a database of all available symbol layer types.

To access symbol layer data, use its properties() method that returns a key-value dictionary of properties which determine the appearance. Each symbol layer type has a specific set of properties that it uses. Additionally, there are the generic methods color(), size(), angle() and width(), with their setter counterparts. Of course size and angle are available only for marker symbol layers and width for line symbol layers.

6.8.4.2. 사용자 지정 심볼 레이어 유형 생성

데이터를 어떻게 렌더링할지 사용자 지정하고 싶다고 상상해보십시오. 사용자가 원하는 방식대로 피처를 그리는, 자신만의 심볼 레이어 클래스를 생성할 수 있습니다. 다음은 지정된 반지름으로 빨간색 원을 그리는 마커의 예시 코드입니다.

from qgis.core import QgsMarkerSymbolLayer
from qgis.PyQt.QtGui import QColor

class FooSymbolLayer(QgsMarkerSymbolLayer):

  def __init__(self, radius=4.0):
      QgsMarkerSymbolLayer.__init__(self)
      self.radius = radius
      self.color = QColor(255,0,0)

  def layerType(self):
     return "FooMarker"

  def properties(self):
      return { "radius" : str(self.radius) }

  def startRender(self, context):
    pass

  def stopRender(self, context):
      pass

  def renderPoint(self, point, context):
      # Rendering depends on whether the symbol is selected (QGIS >= 1.5)
      color = context.selectionColor() if context.selected() else self.color
      p = context.renderContext().painter()
      p.setPen(color)
      p.drawEllipse(point, self.radius, self.radius)

  def clone(self):
      return FooSymbolLayer(self.radius)

The layerType() method determines the name of the symbol layer; it has to be unique among all symbol layers. The properties() method is used for persistence of attributes. The clone() method must return a copy of the symbol layer with all attributes being exactly the same. Finally there are rendering methods: startRender() is called before rendering the first feature, stopRender() when the rendering is done, and renderPoint() is called to do the rendering. The coordinates of the point(s) are already transformed to the output coordinates.

For polylines and polygons the only difference would be in the rendering method: you would use renderPolyline() which receives a list of lines, while renderPolygon() receives a list of points on the outer ring as the first parameter and a list of inner rings (or None) as a second parameter.

일반적으로 사용자가 표현을 변경할 수 있도록 심볼 레이어 유형의 속성을 설정하기 위한 GUI를 추가하는 것이 편리합니다. 앞의 예시에서 사용자가 원의 반경을 설정하도록 할 수 있습니다. 다음은 그런 위젯을 구현하는 예시 코드입니다.

from qgis.gui import QgsSymbolLayerWidget

class FooSymbolLayerWidget(QgsSymbolLayerWidget):
    def __init__(self, parent=None):
        QgsSymbolLayerWidget.__init__(self, parent)

        self.layer = None

        # setup a simple UI
        self.label = QLabel("Radius:")
        self.spinRadius = QDoubleSpinBox()
        self.hbox = QHBoxLayout()
        self.hbox.addWidget(self.label)
        self.hbox.addWidget(self.spinRadius)
        self.setLayout(self.hbox)
        self.connect(self.spinRadius, SIGNAL("valueChanged(double)"), \
            self.radiusChanged)

    def setSymbolLayer(self, layer):
        if layer.layerType() != "FooMarker":
            return
        self.layer = layer
        self.spinRadius.setValue(layer.radius)

    def symbolLayer(self):
        return self.layer

    def radiusChanged(self, value):
        self.layer.radius = value
        self.emit(SIGNAL("changed()"))

This widget can be embedded into the symbol properties dialog. When the symbol layer type is selected in symbol properties dialog, it creates an instance of the symbol layer and an instance of the symbol layer widget. Then it calls the setSymbolLayer() method to assign the symbol layer to the widget. In that method the widget should update the UI to reflect the attributes of the symbol layer. The symbolLayer() method is used to retrieve the symbol layer again by the properties dialog to use it for the symbol.

On every change of attributes, the widget should emit the changed() signal to let the properties dialog update the symbol preview.

이제 마지막 단계만 남았습니다. QGIS가 이 새 클래스들을 인식하도록 만드는 일입니다. 심볼 레이어를 레지스트리에 추가하면 됩니다. 레지스트리에 추가하지 않고 심볼 레이어를 사용할 수도 있지만, 예를 들어 사용자 지정 심볼 레이어를 담고 있는 프로젝트 파일을 불러온다든가 GUI에서 레이어 속성을 편집할 수 없는 등 몇몇 기능들을 사용할 수 없게 됩니다.

심볼 레이어에 대한 메타데이터도 생성해야 합니다.

from qgis.core import QgsSymbol, QgsSymbolLayerAbstractMetadata, QgsSymbolLayerRegistry

class FooSymbolLayerMetadata(QgsSymbolLayerAbstractMetadata):

  def __init__(self):
    super().__init__("FooMarker", "My new Foo marker", QgsSymbol.Marker)

  def createSymbolLayer(self, props):
    radius = float(props["radius"]) if "radius" in props else 4.0
    return FooSymbolLayer(radius)

fslmetadata = FooSymbolLayerMetadata()

QgsApplication.symbolLayerRegistry().addSymbolLayerType(fslmetadata)

You should pass layer type (the same as returned by the layer) and symbol type (marker/line/fill) to the constructor of the parent class. The createSymbolLayer() method takes care of creating an instance of symbol layer with attributes specified in the props dictionary. And there is the createSymbolLayerWidget() method which returns the settings widget for this symbol layer type.

마지막 단계는 이 심볼 레이어를 레지스트리에 추가하는 일입니다. 이제 모두 끝났습니다.

6.8.5. 사용자 정의 렌더러 생성

피처를 렌더링하는 데 어떻게 심볼을 선택할지에 대한 규칙을 마음대로 지정하고 싶을 경우 새로운 렌더러를 만드는 것이 유용할 수도 있습니다. 여러 항목을 조합해서 심볼을 결정해야 하거나, 현재 축척에 따라 심볼 크기를 변경해야 하는 등의 경우에 새로운 렌더러를 만들면 좋습니다.

다음 예시 코드는 마커 심볼 2개를 생성해서 각 피처마다 임의로 2개 중 1개를 선택하는 간단한 사용자 지정 렌더러입니다.

import random
from qgis.core import QgsWkbTypes, QgsSymbol, QgsFeatureRenderer


class RandomRenderer(QgsFeatureRenderer):
  def __init__(self, syms=None):
    super().__init__("RandomRenderer")
    self.syms = syms if syms else [
      QgsSymbol.defaultSymbol(QgsWkbTypes.geometryType(QgsWkbTypes.Point)),
      QgsSymbol.defaultSymbol(QgsWkbTypes.geometryType(QgsWkbTypes.Point))
    ]

  def symbolForFeature(self, feature, context):
    return random.choice(self.syms)

  def startRender(self, context, fields):
    super().startRender(context, fields)
    for s in self.syms:
      s.startRender(context, fields)

  def stopRender(self, context):
    super().stopRender(context)
    for s in self.syms:
      s.stopRender(context)

  def usedAttributes(self, context):
    return []

  def clone(self):
    return RandomRenderer(self.syms)

The constructor of the parent QgsFeatureRenderer class needs a renderer name (which has to be unique among renderers). The symbolForFeature() method is the one that decides what symbol will be used for a particular feature. startRender() and stopRender() take care of initialization/finalization of symbol rendering. The usedAttributes() method can return a list of field names that the renderer expects to be present. Finally, the clone() method should return a copy of the renderer.

Like with symbol layers, it is possible to attach a GUI for configuration of the renderer. It has to be derived from QgsRendererWidget. The following sample code creates a button that allows the user to set the first symbol

from qgis.gui import QgsRendererWidget, QgsColorButton


class RandomRendererWidget(QgsRendererWidget):
  def __init__(self, layer, style, renderer):
    super().__init__(layer, style)
    if renderer is None or renderer.type() != "RandomRenderer":
      self.r = RandomRenderer()
    else:
      self.r = renderer
    # setup UI
    self.btn1 = QgsColorButton()
    self.btn1.setColor(self.r.syms[0].color())
    self.vbox = QVBoxLayout()
    self.vbox.addWidget(self.btn1)
    self.setLayout(self.vbox)
    self.btn1.colorChanged.connect(self.setColor1)

  def setColor1(self):
    color = self.btn1.color()
    if not color.isValid(): return
    self.r.syms[0].setColor(color)

  def renderer(self):
    return self.r

The constructor receives instances of the active layer (QgsVectorLayer), the global style (QgsStyle) and the current renderer. If there is no renderer or the renderer has different type, it will be replaced with our new renderer, otherwise we will use the current renderer (which has already the type we need). The widget contents should be updated to show current state of the renderer. When the renderer dialog is accepted, the widget’s renderer() method is called to get the current renderer — it will be assigned to the layer.

마지막으로 렌더러의 메타데이터를 생성하고 레지스트리에 렌더러를 등록해야 합니다. 이렇게 하지 않으면 렌더러와 함께 레이어를 불러올 수 없고, 사용자가 렌더러 목록에서 렌더러를 선택할 수 없습니다. 이제 RandomRenderer 예시 코드를 완성해봅시다.

from qgis.core import (
  QgsRendererAbstractMetadata,
  QgsRendererRegistry,
  QgsApplication
)

class RandomRendererMetadata(QgsRendererAbstractMetadata):

  def __init__(self):
    super().__init__("RandomRenderer", "Random renderer")

  def createRenderer(self, element):
    return RandomRenderer()

  def createRendererWidget(self, layer, style, renderer):
    return RandomRendererWidget(layer, style, renderer)

rrmetadata = RandomRendererMetadata()

QgsApplication.rendererRegistry().addRenderer(rrmetadata)

Similarly as with symbol layers, abstract metadata constructor awaits renderer name, name visible for users and optionally name of renderer’s icon. The createRenderer() method passes a QDomElement instance that can be used to restore the renderer’s state from the DOM tree. The createRendererWidget() method creates the configuration widget. It does not have to be present or can return None if the renderer does not come with GUI.

To associate an icon with the renderer you can assign it in the QgsRendererAbstractMetadata constructor as a third (optional) argument — the base class constructor in the RandomRendererMetadata __init__() function becomes

QgsRendererAbstractMetadata.__init__(self,
       "RandomRenderer",
       "Random renderer",
       QIcon(QPixmap("RandomRendererIcon.png", "png")))

The icon can also be associated at any later time using the setIcon() method of the metadata class. The icon can be loaded from a file (as shown above) or can be loaded from a Qt resource (PyQt5 includes .qrc compiler for Python).

6.9. 남은 이야기들

TODO:

• creating/modifying symbols

• working with style (QgsStyle)

• working with color ramps (QgsColorRamp)

• exploring symbol layer and renderer registries