Fix mypy type errors and add mypy to .drone.yml

[aucampia]

Fixes #1311

Add mypy to .drone.yml and fix type errors that come up.

Not type checking examples or tests.

Other changes:

• fix return value for Graph.serialize (refs Make graph and other methods chainable #1394)
• remove default value for
  rdflib.plugins.sparql.algebra.translateAlgebra (refs Implemented function translateAlgebra. This functions takes a SPARQL … #1322)
• add .dockerignore to reduce context size and make docker quicker
  to run.
• add .flake8 config to ignore line length as black is managing
  formatting.
• add mypy to docker-compose, makefile and tox.ini
• fix the way csv2rdf is invoked to ensure that the right code gets
  executed.

[aucampia]

@nicholascar This change will make drone fail if mypy fails, if you want I can change it so that errors are discarded (i.e. add || true).

[nicholascar]

Looks sensible to me so over to @ashleysommer / @white-gecko to just check the mypy / drone implications.

[aucampia]

I included the following as per discussion in #1394

Change Graph.serialize to throw for invalid destinations

Graph.serialize was just logging a warning if the supplied destination
was not valid, but it would be better to fail if the method does not
perform the requested operation.

This change makes Graph.serialize throw if the supplied destination
can't be written to.

Don't mind moving this to a separate PR if desired.

[nicholascar]

All good with the inclusions of the error raise in this PR. @ashleysommer can you please take a look?

[ajnelson-nist]

I hope you'll pardon my butting in. I've been trying mypy in some other projects. I've found some cases require the addition of a py.typed file, with some extra setup references needed to export the py.typed file. One place I did this is here. I did this from reviewing PEP 561 and from finding it necessary from some tinkering.

Is it more appropriate for rdflib to have, or not to have, a py.typed file?

[aucampia]

I hope you'll pardon my butting in. I've been trying mypy in some other projects. I've found some cases require the addition of a py.typed file, with some extra setup references needed to export the py.typed file. One place I did this is here. I did this from reviewing PEP 561 and from finding it necessary from some tinkering.

Is it more appropriate for rdflib to have, or not to have, a py.typed file?

I was not aware of this, thanks for the info. I think it should have py.typed in this case then. I will try test this and see what is up, maybe make another PR to add it.

[nicholascar]

From the PEP: “if a top-level package includes it, all its sub-packages MUST support type checking as well.”

So please do go ahead and add a py.typed file but just consider where it should be added. I assume to the rdflib/ folder, not the repo parent folder!

Unless test/ also needs one…

Need to get rid of nose too… Ugh, lots to do…

[aucampia]

So please do go ahead and add a py.typed file but just consider where it should be added. I assume to the rdflib/ folder, not the repo parent folder!

I added it in rdflib/ - and put it on this PR. I think it only really matters for what is distributed in wheels anyway, so even if we enforce typing in test/ it is not needed there as others should not be importing tests.

This will prevent the following error from occuring:

$ python3.9 -m mypy --show-error-context --show-error-codes  test_rdflib.py
test_rdflib.py:1: error: Skipping analyzing "rdflib": found module but no type hints or library stubs  [import]
test_rdflib.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)

test_rdflib.py:

import rdflib

# python3.9 -m mypy --show-error-context --show-error-codes --strict --allow-untyped-calls test_rdflib.py

g = rdflib.Graph()

Thanks again for the heads up on this @ajnelson-nist

[ajnelson-nist]

I'm glad my feedback was helpful!

I look forward to seeing a release with this PR. Adding type checking to my RDF projects still requires # type: ignore on all of my rdflib imports, though I feel confident that won't be needed much longer.

[ashleysommer]

I'm sorry I didn't get a chance to review this before it was merged.
Why was this changed, it has implicates for all Store implementations that don't implement namespaces().

[ashleysommer]

Ok, I just saw #1432, with an explanation.