QGIS Temporary Scratch Layer attribute name become `:(0,0)`

Question

I wrote a simple plugin to retrieve and synchronize points from remote MongoDb into a generated scratch layer with all the attributes every time the project open, edit and commit but the attribute naming bothering me for sometimes.

Scratch layer is in-memory layer that does not preserve the features upon exit. However I can save the project together with the layer.

Whenever I load the project, the attributes name of scratch layer gets appended with :(0,0). E.g. the attribute changed from city to city:(0,0) why?

Edit:

Apparently it will only happened if the attributes created using python.

How to produce:

layer = iface.activeLayer()
from PyQt4.QtCore import QVariant
field = QgsField("test", QVariant.String)
layer.dataProvider().addAttributes([field])
layer.updateFields()

1. Create Temporary Scratch Layer
2. Run the code above in Python Console
3. Save the layer edit and project into a .qgs file.
4. Exit and reopen QGIS
5. Reopen the saved project file
6. Open the layer Properties > Fields
7. The attribute name test will turns into test:(0,0)

I am using QGIS 2.18.2

Edit2:

I found that QGIS does not preserve the data type string into the project file when the field created using python. The project file .qgs xml content under qgs > projectlayers > maplayer > datasource is:

memory?geometry=None&field=test:(0,0)

but it should be:

memory?geometry=None&field=test:string(0,0)

Answer 1

Short answer

It needs to be specific of the third parameter, typeName like this:

QgsField("fieldName", QVariant.String, "string")

So that it will be properly serialized/deserialized in project file.

Long answer

Note the typeName is provider specific type unique to the source. It can be "string", "integer" or "double" based on the documentation. I tried "datetime" seems to work because that is a memory provider.

In order to know what is the typeName other than that, try to create the field manually (trough layer Properties > Fields), save and check the content of .qgs project file where the memory layer<datasource> defined.

Reference:

1. Adding and Removing Fields and using Memory Provider was documented here but only use first two parameters of QgsField constructor.

2. The above link and c++ QgsVectorLayer class detailed description also mentioned about the similar format defined in the project file:

field=name:type(length,precision) Defines an attribute of the layer. Multiple field parameters can be added to the data provider definition. type is one of "integer", "double", "string".

3. The c++ QgsField class constructor shows about the typeName but does gives only SQL like example eg. text or varchar instead of string. I believe typeName is anything that the provider support. You can just try to save a data type and check the .qgs file like I mention above.

Issue:

This is definitely an issue, the field type information does not get serialized/deserialized properly into project file. Apart from that, the memory provider should also translate the QVariant.String into "string" or maybe state in documentation, the typeName parameter is compulsory.

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