I am trying create new columns in an attribute table and fill them with values with expressions using python console in QGIS.

I successfully create the new columns but when I try to fill them, I get an assertion error. Here is an example for one column :

layer=QgsVectorLayer(r"C:\Users\ANI\Documents\SIG\fossés\pythontest\mergedlinef1.shp")
features=layer.getFeatures()

layer_provider=layer.dataProvider()
layer_provider.addAttributes([QgsField("long",QVariant.Double)])
layer.updateFields()

layer.startEditing()
exp_long = QgsExpression("$length")
context = QgsExpressionContext()
context.appendScopes(QgsExpressionContextUtils.globalProjectLayerScopes(layer))

with edit(layer):
    for f in layer.getFeatures():
        context.SetFeature(f)
        f['long'] = exp_long.evaluate(context)
        layer.updateFeature(f)

And the error :

Traceback (most recent call last):
  File "C:\PROGRA~1\QGIS32~1.0\apps\Python39\lib\code.py", line 90, in runcode
    exec(code, self.locals)
  File "<input>", line 1, in <module>
  File "<string>", line 23, in <module>
  File "C:\PROGRA~1/QGIS32~1.0/apps/qgis/./python\qgis\core\additions\edit.py", line 38, in __enter__
    assert self.layer.startEditing()
AssertionError

Line 23 refers to the line with edit(layer):. What am I doing wrong ?

I am trying create new columns in an attribute table and fill them with values with expressions using Python Console in QGIS.

I successfully create the new columns but when I try to fill them, I get an assertion error. Here is an example for one column :

layer=QgsVectorLayer(r"C:\Users\ANI\Documents\SIG\fossés\pythontest\mergedlinef1.shp")
features=layer.getFeatures()

layer_provider=layer.dataProvider()
layer_provider.addAttributes([QgsField("long",QVariant.Double)])
layer.updateFields()

layer.startEditing()
exp_long = QgsExpression("$length")
context = QgsExpressionContext()
context.appendScopes(QgsExpressionContextUtils.globalProjectLayerScopes(layer))

with edit(layer):
    for f in layer.getFeatures():
        context.SetFeature(f)
        f['long'] = exp_long.evaluate(context)
        layer.updateFeature(f)

And the error :

Traceback (most recent call last):
  File "C:\PROGRA~1\QGIS32~1.0\apps\Python39\lib\code.py", line 90, in runcode
    exec(code, self.locals)
  File "<input>", line 1, in <module>
  File "<string>", line 23, in <module>
  File "C:\PROGRA~1/QGIS32~1.0/apps/qgis/./python\qgis\core\additions\edit.py", line 38, in __enter__
    assert self.layer.startEditing()
AssertionError

Line 23 refers to the line with edit(layer):. What am I doing wrong ?