PyQGIS – Code Works in Python IDE but Not in QGIS Python Editor

pyqgisqgis-3spyder

I have a working python code that runs within the SPYDER IDE, but doesn't work when run from inside the Python console in QGIS. I have checked the QGIS installs and I can see both panda and shapely. The code opens a .csv (extension .mcsv by the creator) and creates a Polygon from it.

from PyQt5 import QtWidgets
import pandas as pd
from shapely.geometry import Polygon


import_file_path, _ = QtWidgets.QFileDialog.getOpenFileName(
    None, "Select File", "", "Line Files (*.mcsv)")
if import_file_path == '':
    msgBox = QtWidgets.QMessageBox()
    msgBox.setText("No file selected")
    msgBox.setStandardButtons(QtWidgets.QMessageBox.Ok)
    msgBox.setIcon(QtWidgets.QMessageBox.Warning)
    msgBox.exec_()


data = pd.read_csv(import_file_path)

df = pd.DataFrame(data, columns=['Easting', 'Northing'])
df.drop(df.tail(1).index, inplace=True)  # drop -9999 row
df['Easting'] = df['Easting'].astype("string")
df['Northing'] = df['Northing'].astype("string")


p = Polygon(list(zip(df["Easting"], df["Northing"])))

print(str(p))

The .mcsv looks like this:

Map,Name,Easting,Northing
1,1,416213.873,6507344.904
1,1,424283.100,6509982.331
1,1,428632.580,6496675.102
1,1,418176.907,6493257.665
1,1,415338.984,6501940.281
1,1,416213.873,6507344.904
-9999,-9999,-9999,-9999

and the SPYDER generated output looks like:

POLYGON ((416213.873 6507344.904, 424283.1 6509982.331, 428632.58 6496675.102, 418176.907 6493257.665, 415338.984 6501940.281, 416213.873 6507344.904))

And this is the QGIS error message it get:

exec(Path('C:/Users/client/AppData/Local/Temp/tmpq5jtxh23.py').read_text())
Traceback (most recent call last):
  File "C:\PROGRA~1\QGIS32~1.2\apps\Python39\lib\code.py", line 90, in runcode
    exec(code, self.locals)
  File "<input>", line 1, in <module>
  File "<string>", line 24, in <module>
  File "C:\PROGRA~1\QGIS32~1.2\apps\Python39\lib\site-packages\shapely\geometry\polygon.py", line 261, in __init__
    ret = geos_polygon_from_py(shell, holes)
  File "C:\PROGRA~1\QGIS32~1.2\apps\Python39\lib\site-packages\shapely\geometry\polygon.py", line 539, in geos_polygon_from_py
    ret = geos_linearring_from_py(shell)
  File "C:\PROGRA~1\QGIS32~1.2\apps\Python39\lib\site-packages\shapely\geometry\polygon.py", line 502, in geos_linearring_from_py
    lgeos.GEOSCoordSeq_setX(cs, i, coords[0])
ctypes.ArgumentError: argument 4: <class 'TypeError'>: wrong type

QGIS 3.24.2 – Tisler Python version 3.9.5

Best Answer

You are converting your coordinates to "string" type:

df['Easting'] = df['Easting'].astype("string")
df['Northing'] = df['Northing'].astype("string")

and then making a polygon:

p = Polygon(list(zip(df["Easting"], df["Northing"])))

A simple test in Python (outside QGIS) gives me this:

>>> df = pd.DataFrame(dict(Easting=[1,2,3,4],Northing=[4,5,6,7]))
>>> df['Easting'] = df['Easting'].astype("string")
>>> df['Northing'] = df['Northing'].astype("string")
>>> p = Polygon(list(zip(df["Easting"], df["Northing"])))
Traceback (most recent call last):
  File "shapely/speedups/_speedups.pyx", line 252, in shapely.speedups._speedups.geos_linearring_from_py
AttributeError: 'list' object has no attribute '__array_interface__'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/rowlings/.local/lib/python3.8/site-packages/shapely/geometry/polygon.py", line 261, in __init__
    ret = geos_polygon_from_py(shell, holes)
  File "/home/rowlings/.local/lib/python3.8/site-packages/shapely/geometry/polygon.py", line 539, in geos_polygon_from_py
    ret = geos_linearring_from_py(shell)
  File "shapely/speedups/_speedups.pyx", line 373, in shapely.speedups._speedups.geos_linearring_from_py
TypeError: must be real number, not str

without the type conversion it works fine:

>>> df = pd.DataFrame(dict(Easting=[1,2,3,4],Northing=[4,5,6,7]))
>>> p = Polygon(list(zip(df["Easting"], df["Northing"])))
>>>

I don't know why this works for you in Spyder, perhaps a different version of shapely that tolerates string inputs and does a conversion to numeric?

Related Solutions

pyqgis – How to Create an Auto Increment ID Field in PyQGIS

Firstly, regarding the errors you are getting. Here you are storing the layer Data Provider object in the pr variable:

pr = vl.dataProvider()

Then in these lines:

pr.dataProvider().addAttributes(fields)
pr.updateFields()

You are calling dataProvider() and updateFields() on the QgsVectorDataProvider object, however, these methods come from the QgsMapLayer and QgsVectorLayer classes, so both will cause errors.

If your goal is to update your "fid" field all at once after you have added some features, you can just do this:

with edit(vl):
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        f['fid'] = f.id()
        # Or if you want your fid to start from zero instead of 1...
        #f['fid'] = f.id()-1
        vl.updateFeature(f)

However, you should know that this approach you are attempting will not have any any effect at the time a feature is added, it will simply update the "fid" column for already existing features.

It sounds to me like what you really want is to set a default value for your "fid" field during the layer creation using pyqgis, so that when any feature is added, the "fid" field is automatically filled with incrementing integers. In that case, you can use the following code:

vl = QgsVectorLayer("Polygon?crs=epsg:28355", "blank_poly", "memory")
pr = vl.dataProvider()
pr.addAttributes( [ QgsField("fid", QVariant.Int),
                QgsField("Label",  QVariant.String,'character', 255),
                QgsField("Source", QVariant.String),
                QgsField("Date", QVariant.Date),
                QgsField("Status", QVariant.String),
                QgsField("Theme", QVariant.String),
                QgsField("SubTheme", QVariant.String),
                QgsField("Comments", QVariant.String,'character', 255),
                QgsField("Area", QVariant.String),
                QgsField("Name", QVariant.String)] )
                
vl.updateFields()

default_val = QgsDefaultValue(
                """
                CASE
                WHEN maximum("fid") is NULL THEN 1
                WHEN "fid" is NULL THEN maximum("fid")+1
                ELSE "fid"
                END
                """,
                applyOnUpdate=True)

vl.setDefaultValueDefinition(vl.fields().lookupField('fid'), default_val)

QgsProject.instance().addMapLayer(vl)
vl.startEditing()

Again, if you want your "fid" to start from zero instead of one, change the expression to:

"""
CASE
WHEN maximum("fid") is NULL THEN 0
WHEN "fid" is NULL THEN maximum("fid")+1
ELSE "fid"
END
"""

PyQGIS – How to Run a QGIS Plugin on Its Python API

It looks like you are trying to run a gui based Python plugin as a processing algorithm, which simply isn't going to work. To use a processing.run() call like this, you must pass the method an actual processing algorithm which has been added to the processing toolbox by a processing provider (either native or 3rd party).

While it is technically possible to programmatically access the classes and methods of an installed Python plugin, for example the following will open the dialog of the plugin:

plugin = qgis.utils.plugins['delete_null_fields']
# print(plugin)
plugin.run()

how useful this actually is depends very much on the structure of the plugin.

In this instance, to achieve what you want, I think it is easier to write our own script.

Fortunately, we can write a fairly quick and simple script containing a function which will create a copy of the input layer, delete any fields with all NULL values and save the output to a specified folder, leaving the original input layer unchanged. We can then iterate over all the project layers, skipping any non-vector layers, and call our function, passing the layer object, the output path and output format as arguments.

Paste the script below into a new editor in the Python console, edit the output path and click run.

import os

# Define a function with a few parameters to do what what we want

def removeNullColumns(layer, output_dir, file_format):
    # Create an in memory copy of the input layer
    temp_layer = layer.materialize(QgsFeatureRequest())
    # Compile list of fields with all NULL values
    fld_ids = []
    for fld in temp_layer.fields():
        contains_values = False
        for ft in temp_layer.getFeatures():
            if ft[fld.name()] != NULL:
                contains_values = True
                break
        if not contains_values:
            fld_ids.append(temp_layer.fields().lookupField(fld.name()))
    
    # Delete the null fields and update the temp layer fileds
    temp_layer.dataProvider().deleteAttributes(fld_ids)
    temp_layer.updateFields()
    
    # Save the temp layer with input layer name appended with '_no_null'
    # to the specified output folder in the specified format
    save_params = {'INPUT':temp_layer,
                    'OUTPUT':os.path.join(output_dir, f'{layer.name()}_no_null{file_format}')}
    processing.run("native:savefeatures", save_params)
    # If you want to load all the output layer, replace the line above with:
    # processing.runAndLoadResults("native:savefeatures", save_params)

# Define path to your output folder location
destination_folder = '/path/to/output/folder'# ******************EDIT THIS LINE

# Specify the desired output format. If you want Geopackage change to '.gpkg'
output_format = '.shp'

# Iterate over loaded layers, skipping any non-vector layers
# and call the removeNullColumns() function for each one
for lyr in QgsProject.instance().mapLayers().values():
    if not isinstance(lyr, QgsVectorLayer):
        continue
    removeNullColumns(lyr, destination_folder, output_format)
print('Done')

Best Answer

Related Solutions

pyqgis – How to Create an Auto Increment ID Field in PyQGIS

PyQGIS – How to Run a QGIS Plugin on Its Python API

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