Add suport for python-brace-format

Building upon python-babel#838, this adds support for python-brace-format flags in
the python extractor.

Fixes python-babel#333

babel/messages/extract.py

@@ -22,7 +22,7 @@
 import sys
 from tokenize import generate_tokens, COMMENT, NAME, OP, STRING
 
-from babel.util import parse_encoding, parse_future_flags, pathmatch, has_python_format
+from babel.util import parse_encoding, parse_future_flags, pathmatch, has_python_format, has_python_brace_format
 from textwrap import dedent
 
 
@@ -480,6 +480,9 @@ def extract_python(fileobj, keywords, comment_tags, options):
                 if has_python_format(message for message in messages if message):
                     flags.add("python-format")
 
+                if has_python_brace_format(message for message in messages if message):
+                    flags.add('python-brace-format')
+
                 if len(messages) > 1:
                     messages = tuple(messages)
                 else:

babel/util.py

@@ -16,6 +16,7 @@
 import re
 import textwrap
 import pytz as _pytz
+import string
 from babel import localtime
 
 missing = object()
@@ -281,3 +282,16 @@ def has_python_format(ids):
     if isinstance(ids, str):
         ids = [ids]
     return any(PYTHON_FORMAT.search(id) for id in ids)
+
+
+FORMATTER = string.Formatter()
+
+
+def has_python_brace_format(ids):
+    if isinstance(ids, str):
+        ids = [ids]
+    return any(
+        field_name
+        for message_id in ids
+        for _, field_name, format_spec, conversion in FORMATTER.parse(message_id)
+    )

tests/test_util.py

@@ -18,7 +18,7 @@
 import pytest
 
 from babel import util
-from babel.util import parse_future_flags, has_python_format
+from babel.util import parse_future_flags, has_python_format, has_python_brace_format
 
 
 class _FF:
@@ -124,6 +124,23 @@ def test_has_python_format(ids):
 
 @pytest.mark.parametrize('ids', [
     ('foo',),
+    ('foo {name}',),
 ])
 def test_not_has_python_format(ids):
     assert not has_python_format(ids)
+
+@pytest.mark.parametrize('ids', [
+    ('foo {name} bar',),
+    ('foo {name:.3f} bar',),
+    ('foo {name!r:20} bar',),
+])
+def test_has_python_brace_format(ids):
+    assert has_python_brace_format(ids)
+
+@pytest.mark.parametrize('ids', [
+    ('foo',),
+    ('fo {}', ),
+    ('foo %d bar',),
+])
+def test_not_has_python_brace_format(ids):
+    assert not has_python_brace_format(ids)