How to concatenate strings separated by dot in PyQGIS?

Question

I would like to concatenate the string of field1 with field2 separated by dot and so on. However, when I put the "." the attribute values remain null. How to separate string by dot?

e = QgsExpression('concat((lpad("field1",2,0)),".",(lpad("field2",2,0)),".",(lpad("field3",3,0)),".",(lpad("field4",4,0)))')

Answer 1

I did some tests (and some fails) on this code snippet:

layer=iface.activeLayer() # load the layer as you want
for feat in layer.getFeatures():
    e = QgsExpression('concat((lpad("field1",2,0)),".",(lpad("field2",2,0)),".",(lpad("field3",3,0)),".",(lpad("field4",4,0)))')
    value = e.evaluate()

before finding the solution, assuming that you are working on a layer with some fields ("field1", "field2", and so on).

Firstly, you can't call the fields in that way because they are not read correctly: writing "field1" as is doesn't provide its value but rather a string that stores the "field1" text. In fact, using the code snippet above with your expression would print:

print value
[f[.][f[.][fi[.][fie

which doesn't make any sense. So, the first step was slightly changing the sample code:

layer=iface.activeLayer() # load the layer as you want
for feat in layer.getFeatures():
    expr = 'concat((lpad(%s,2,0)),".",(lpad(%s,2,0)),".",(lpad(%s,3,0)),".",(lpad(%s,4,0)))' %(feat["field1"], feat["field2"], feat["field3"], feat["field4"])
    e = QgsExpression(expr)
    value = e.evaluate()

I thought it would be enough, but printing the value variable gave me:

print value
01[.]06[.]015[.]0008

I solved the issue by using the char() function instead of "." (or '.'). The char() function, available from the Field Calculator, returns the character associated with a Unicode code. For the (full stop) dot, the Unicode code is 46 (see this Wikipedia page for reference). This was the final code:

layer=iface.activeLayer() # load the layer as you want
for feat in layer.getFeatures():
    expr = 'concat((lpad(%s,2,0)),char(46),(lpad(%s,2,0)),char(46),(lpad(%s,3,0)),char(46),(lpad(%s,4,0)))' %(feat["field1"], feat["field2"], feat["field3"], feat["field4"])
    e = QgsExpression(expr)
    value = e.evaluate()

which printed the expected result:

print value
01.06.015.0008

---

Bonus If you want to store the result in a new field (e.g. "field5"), adding these lines to the last code should be enough for updating it:

layer=iface.activeLayer() # load the layer as you want
for feat in layer.getFeatures():
    expr = 'concat((lpad(%s,2,0)),char(46),(lpad(%s,2,0)),char(46),(lpad(%s,3,0)),char(46),(lpad(%s,4,0)))' %(feat["field1"], feat["field2"], feat["field3"], feat["field4"])
    e = QgsExpression(expr)
    value = e.evaluate()
    with edit(layer):
        feat["field5"] = value
        layer.updateFeature(feat)

Answer 2

Strings in expressions need to be wrapped in single quotation marks, i.e.

concat((lpad("field1",2,0)),'.',
       (lpad("field2",2,0)),'.',
       (lpad("field3",3,0)),'.',
       (lpad("field4",4,0)))