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DIY_GIT_in_Python/how_to/Change_27.md

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- k: Iterate commits and parents
In addition to printing the refs, we'll also print all OIDs that are reachable
from those refs. We'll create `iter_commits_and_parents`, which is a generator
that returns all commits that it can reach from a given set of OIDs.
Note that `iter_commits_and_parents` will return an OID once, even if it's
reachable from multiple refs. Here, for example:
```
o<----o<----o<----o<----@<----@<----@
^ \ ^
first commit -<--$<----$ refs/tags/tag1
^
refs/tags/tag2
```
We can reach the first commit by following the parents of *tag1* or by following
the parents of *tag2*. Yet if we call `iter_commits_and_parents({tag1, tag2})`,
the first commit will be yielded only once. This property will be useful later.
(Note that nothing is visualized yet, we're preparing for that.)