First of all, all commits should be atomic, that is they shouldn't include unrelated changes. Fixing a typo or spacing while fixing bug in related code is acceptable, but fixing 6 bugs and adding 2 features in the same commit makes it hard for people to parse out what change was for in the future. A good rule of thumb is that if a summary of your changes can't fit in one line, it's probably too big.
The first line of the commit message is most important part. This is especially true today, where many DVCSes only show the first line of the commit by default in their log command. The summary line should succinctly summarize what your change is and what it accomplishes. It need not be a full sentence, but just a bug number or general statement ("fix this") is not appropriate. The best summary lines quickly inform any log browser of the purpose and changes in the commit. Summary lines should also never be wrapped. Nothing is more annoying than reading a summary line which is cut off in the middle by a line break. Simple typo fixes do not require complicated messages. Good examples:
fix #2345 by preventing add() from accepting strings
fix a segfault in foo_my_bars() #4563
add a Python interface to the tokenizer #3222
and bad ones:
test and a fix
a huge change to Foo class
why does this not work?
After the summary line can optionally come a body. A blank line should always separate the commit message from the body and different sections of the body from another. Bodies should also always be line wrapped. The body can include any of the following:
- Bullet points describing various aspect of the change in more detail.
- A paragraph description explaining why how something was implemented or why it's written a certain way.
- A reference to mailing list discussions or decisions that lead to the commit.
- Authors and attributions.
- Any other significant information about the commit. For example, explain how it affects external components or might result in unexpected behavior.
Some projects follow the convention of listing affected files in bullet points and describing the individual changes to each. I personally find a prose summary of the changes in the body along with a diff or the verbose version of the log which shows changed files more helpful than this technique.
Good examples of complete commit messages:
normalize encoding before opening file #3242
This change requires that tokenizer.c be linked with the Unicode
silence foo warnings by default
Approved by BDFL in
support unicode in shlex module #4523
This is implemented by providing a separate class for Unicode and
requiring a locale to be set before parsing commences.
Patch by J. Hacker and J. Programmer
boost the speed of keyword argument comparisons
This improves some function calls by over 30% by comparing for
identity before falling back to the regular comparison. stringobject.c
was modified to provide faster access to a string's value.