Refactor legacy `indent: int` -> `indent: str`

The type of `indent` parameter is preferably `str`. See
http://simplejson.readthedocs.io/en/latest/index.html?highlight=dump#simplejson.dump
. This change allows to specify TABs as indentation delimiter to
`pretty_format_json`. Add input validator/converter for backward compat.

pre_commit_hooks/pretty_format_json.py

@@ -24,6 +24,25 @@ def _autofix(filename, new_contents):
         f.write(new_contents)
 
 
+def parse_indent(s):
+    # type: (str) -> str
+    try:
+        int_indentation_spec = int(s)
+        if int_indentation_spec >= 0:
+            return int_indentation_spec * ' '
+        else:
+            raise ValueError(
+                'Negative integer supplied to construct JSON indentation delimiter. ',
+            )
+    except ValueError:
+        if s.strip() == '':
+            return s
+        else:
+            raise ValueError(
+                'Non-whitespace JSON indentation delimiter supplied. ',
+            )
+
+
 def pretty_format_json(argv=None):
     parser = argparse.ArgumentParser()
     parser.add_argument(
@@ -34,9 +53,9 @@ def pretty_format_json(argv=None):
     )
     parser.add_argument(
         '--indent',
-        type=int,
-        default=2,
-        help='Number of indent spaces used to pretty-format files',
+        type=parse_indent,
+        default='  ',
+        help='String used as delimiter for one indentation level',
     )
     parser.add_argument(
         '--no-sort-keys',

testing/resources/tab_pretty_formatted_json.json

@@ -0,0 +1,9 @@
+{
+	"alist": [
+		2,
+		34,
+		234
+	],
+	"blah": null,
+	"foo": "bar"
+}

tests/pretty_format_json_test.py

@@ -2,10 +2,21 @@ import io
 
 import pytest
 
+from pre_commit_hooks.pretty_format_json import parse_indent
 from pre_commit_hooks.pretty_format_json import pretty_format_json
 from testing.util import get_resource_path
 
 
+def test_parse_indent():
+    assert parse_indent('0') == ''
+    assert parse_indent('2') == '  '
+    assert parse_indent('\t') == '\t'
+    with pytest.raises(ValueError):
+        parse_indent('a')
+    with pytest.raises(ValueError):
+        parse_indent('-2')
+
+
 @pytest.mark.parametrize(('filename', 'expected_retval'), (
     ('not_pretty_formatted_json.json', 1),
     ('unsorted_pretty_formatted_json.json', 1),
@@ -26,6 +37,17 @@ def test_unsorted_pretty_format_json(filename, expected_retval):
     assert ret == expected_retval
 
 
+@pytest.mark.parametrize(('filename', 'expected_retval'), (
+    ('not_pretty_formatted_json.json', 1),
+    ('unsorted_pretty_formatted_json.json', 1),
+    ('pretty_formatted_json.json', 1),
+    ('tab_pretty_formatted_json.json', 0),
+))
+def test_tab_pretty_format_json(filename, expected_retval):
+    ret = pretty_format_json(['--indent', '\t', get_resource_path(filename)])
+    assert ret == expected_retval
+
+
 def test_autofix_pretty_format_json(tmpdir):
     srcfile = tmpdir.join('to_be_json_formatted.json')
     with io.open(get_resource_path('not_pretty_formatted_json.json')) as f: