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Apply unified diffs in memory, compatible with GNU patch.

A quick example:

>>> import textwrap
>>> import unipatch
>>> patch = textwrap.dedent(
...     """\
...     @@ -1,2 +1,3 @@
...      apples
...      bananas
...     +cherries
...     """
... )
>>> print(unipatch.apply_patch("apples\nbananas\n", patch))
apples
bananas
cherries

Installation

Use pip:

python -m pip install unipatch

Python 3.10 to 3.15 supported.

API

apply_patch(source: str, patch: str, forwards: bool=True)

Apply the unified diff in the string patch to the string source and return the resulting string. Pass forwards=False to apply the patch in reverse, undoing it instead.

Raises PatchError if the patch cannot be parsed or does not apply.

>>> unipatch.apply_patch("a\nb\n", "@@ -1 +1 @@\n-a\n+c\n")
'c\nb\n'
>>> unipatch.apply_patch("c\nb\n", "@@ -1 +1 @@\n-a\n+c\n", forwards=False)
'a\nb\n'

PatchError

The base exception class, a subclass of ValueError. It has two subclasses:

  • PatchParseError: the patch is not valid unified diff format.

  • HunkApplyError: the patch parsed, but a hunk’s lines do not match the source.

Exception messages name the failing hunk and what went wrong.

Behaviour

unipatch follows the behaviour of the GNU patch commandline utility, verified by differentially testing against it with randomized inputs:

  • Hunks may apply at an offset from the line numbers stated in their @@ headers, matching by content, with a found offset carrying forward to later hunks.

  • A fuzz factor ignores up to two mismatching context lines at the edges of a hunk. Ignored context lines are left as they are in the source. A hunk with less context on one side, as diff produces at the start or end of a file, only applies at that edge of the source.

  • Oddities are tolerated like GNU patch:

    • Hunks cut short at the end of the patch

    • Context lines that have lost their leading space (empty and tab-led lines)

    • \ No newline at end of file markers

Only unified diff format is supported, not the older context or ed formats. Hence the name is unipatch.

Project links

Data verified by PyPI on Aug 19, 2026
Data provided by the project maintainers, verified at the time the release was uploaded to PyPI.

Key dates

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  • Released:
Latest release

1 maintainer

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Avatar for AdamChainz from gravatar.com AdamChainz

Credits

Author: Adam Johnson

GitHub Statistics

Data verified by PyPI on Aug 19, 2026
The GitHub source repository was provided by the project maintainers and verified by PyPI at the time of upload. Stars, forks, and open issues/PRs are derived from that repository and have not been independently verified.

License expression

MIT
View SPDX License List

Requires

Python >=3.10

Tags

diff patch unified diff

Classifiers

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