add nbqa.ini config file (#79)

nbqa/__main__.py

@@ -1,4 +1,5 @@
 import argparse
+import configparser
 import os
 import re
 import subprocess
@@ -253,6 +254,11 @@ def main(raw_args=None):
             replace_magics.main(temp_python_file)
             _create_blank_init_files(notebook, tmpdirname)
 
+        config = configparser.ConfigParser()
+        config.read(".nbqa.ini")
+        if command in config.sections():
+            configs = [[f"--{key}", val] for key, val in config[command].items()]
+            kwargs.extend([j for i in configs for j in i])
         out, err, output_code = _run_command(
             command, root_dir, tmpdirname, nb_to_py_mapping, kwargs
         )

tests/test_nbqa_ini.py

@@ -0,0 +1,36 @@
+import difflib
+from pathlib import Path
+
+import pytest
+
+from nbqa.__main__ import main
+
+
+def test_nbqa_ini_works(tmp_notebook_for_testing, capsys):
+    """
+    Check .nbqa.ini config is picked up works.
+    """
+
+    with open(".nbqa.ini", "w") as f:
+        f.write("[flake8]\nignore=F401\nselect=E303\n")
+
+    # check diff
+    with open(tmp_notebook_for_testing, "r") as handle:
+        before = handle.readlines()
+    with pytest.raises(SystemExit):
+        main(["flake8", ".", "--ignore", "E302"])
+
+    Path(".nbqa.ini").unlink()
+
+    with open(tmp_notebook_for_testing, "r") as handle:
+        after = handle.readlines()
+    result = "".join(difflib.unified_diff(before, after))
+    expected = ""
+    assert result == expected
+
+    # check out and err
+    out, err = capsys.readouterr()
+    expected_out = "tests/data/notebook_starting_with_md.ipynb:cell_3:0:1: E303 too many blank lines (3)\n"
+    expected_err = ""
+    assert sorted(out.splitlines()) == sorted(expected_out.splitlines())
+    assert sorted(err.splitlines()) == sorted(expected_err.splitlines())