fix joblib model loading

[amfonelic]

Hi! I faced the problem that some models that were dumped using joblib cannot be loaded using pickle in m2cgen i.e. DecisionTreeClassifier from scikit-learn. This leads to undefined behavior further down in the code.

Works file:

from pickle import dump
f = open("model.pkl","wb")
clf = DecisionTreeClassifier()
clf = clf.fit(X_train,y_train)
dump(clf, f) 

Won't work:

from joblib import dump
clf = DecisionTreeClassifier()
clf = clf.fit(X_train,y_train)
dump(clf, "model.pkl") 

Traceback looks like:

Traceback (most recent call last):
  File "~/miniconda3/bin/m2cgen", line 8, in <module>
    sys.exit(main())
  File "~/miniconda3/lib/python3.9/site-packages/m2cgen/cli.py", line 137, in main
    print(generate_code(args))
  File "~/miniconda3/lib/python3.9/site-packages/m2cgen/cli.py", line 132, in generate_code
    return exporter(model, **kwargs)
  File "~/miniconda3/lib/python3.9/site-packages/m2cgen/exporters.py", line 33, in export_to_java
    return _export(model, interpreter)
  File "~/miniconda3/lib/python3.9/site-packages/m2cgen/exporters.py", line 458, in _export
    assembler_cls = get_assembler_cls(model)
  File "~/miniconda3/lib/python3.9/site-packages/m2cgen/assemblers/__init__.py", line 147, in get_assembler_cls
    raise NotImplementedError(f"Model '{model_name}' is not supported")
NotImplementedError: Model 'numpy_ndarray' is not supported

By the way, I'm not the only one with this problem.
#287

[izeigerman]

Unconditional loading of whatever module name is supplied into the CLI feels like a security concern to me. Can we perhaps maintain a list of supported libraries and check whether the supplied argument matches one of them?

[izeigerman]

Can we be a little more specific? Perhaps pickle_lib?

[izeigerman]

Thank you so much for submitting this PR! A few comments, but is good to go otherwise.

[amfonelic]

Thanks for review! Fixed issues you mentioned. Glad to be of service, you make brilliant software :)

[izeigerman]

@pinktoxin thanks for addressing comments! Would you mind fixing the following linter errors please:

./tests/test_cli.py:126:1: E302 expected 2 blank lines, found 1
./tests/test_cli.py:132:1: E302 expected 2 blank lines, found 1
./m2cgen/cli.py:106:22: E231 missing whitespace after ','

[izeigerman]

Thanks for addressing comments! 🚀