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I used the following code to add a separate column to the attribute table with the length of features.

from PyQt4.QtCore import QVariant
from qgis.core import QgsField, QgsExpression, QgsFeature
vl.startEditing()
myField = QgsField( 'myattr', QVariant.Int )
vl.addAttribute( myField )
idx = vl.fieldNameIndex( 'myattr' )
e = QgsExpression( '$length' )
e.prepare( vl.pendingFields() )
for f in vl.getFeatures():
    f[idx] = e.evaluate( f )
    vl.updateFeature( f )
vl.commitChanges()

But sometimes it will add a new column and show 0 or NULL in each row, and sometimes it shows the length of feature in degrees.

What did I do wrong?

And is there any way to add length in meters without changing the project CRS?

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