git: add function to parse message trailers

This is a pure python implementation of the message trailer parsing
algorithm in git (and libgit2). It is intended for use on finalized
commit messages only. Lines starting with comments are not ignored.

The function returns a list of pairs (name, value) where name is the
name of the trailer. Trailer values may span over multiple lines.

Signed-off-by: Robin Jarry <>
Robin Jarry 1 year ago committed by Drew DeVault
parent 060b91237b
commit a9f72b5a59

@ -0,0 +1,83 @@
import re
from typing import List, Tuple
_git_generated_prefixes = (
"Signed-off-by: ",
"(cherry picked from commit ",
def commit_trailers(message: str) -> List[Tuple[str, str]]:
Extract the trailers from a commit message. Return a list of pairs of
(name, value).
This borrows a large amount of logic from git core (trailer.c).
lines = message.strip().splitlines()
# The first paragraph is the title and cannot be trailers
while lines and lines[0] != '':
del lines[0]
recognized_prefix = False
only_spaces = True
trailer_lines = non_trailer_lines = 0
possible_continuation_lines = 0
# Get the start of the trailers by looking starting from the end for a
# blank line before a set of non-blank lines that (i) are all trailers, or
# (ii) contains at least one Git-generated trailer and consists of at least
# 25% trailers.
i = len(lines) - 1
while i >= 0:
line = lines[i]
if not line.strip():
# blank line
if only_spaces:
i -= 1
if recognized_prefix and trailer_lines * 3 >= non_trailer_lines:
i += 1
if trailer_lines > 0 and non_trailer_lines == 0:
i += 1
return []
only_spaces = False
if any(line.startswith(p) for p in _git_generated_prefixes):
trailer_lines += 1
possible_continuation_lines = 0
recognized_prefix = True
elif"^[A-Za-z\d][A-Za-z\d-]*\s*:", line):
trailer_lines += 1
possible_continuation_lines = 0
elif line[0] in (" ", "\t"):
possible_continuation_lines += 1
non_trailer_lines += 1 + possible_continuation_lines
possible_continuation_lines = 0
i -= 1
# Iterate over all remaining lines and collect trailer names and values.
# If a line does not match a trailer and starts with a space or tab, its
# contents are appended to the current trailer value.
trailers = []
name = value = None
for line in lines[i:]:
match = re.match(r"^([A-Za-z\d][A-Za-z\d-]*)\s*:\s*(.*)$", line)
if match:
if name is not None and value is not None:
trailers.append((name, value))
name = match[1]
value = match[2]
elif name is not None and value is not None and line[0] in (" ", "\t"):
# continuation line
value += "\n" + line
if name is not None and value is not None:
trailers.append((name, value))
return trailers