ImportError: cannot import name 'gcd' from 'fractions'

Not specified by the original author.

TL;DR: Python 3.9 removed fractions.gcd. Change the import to from math import gcd, or upgrade the package that does the old import.

The error

ImportError: cannot import name 'gcd' from 'fractions'

Fix it

1. Replace the import in your code:

   from math import gcd

   Expected: identical behavior; math.gcd exists since Python 3.5.

2. If the bad import is inside a third-party package (old snakemake, old networkx), upgrade that package:

   pip install -U [the-package]

   Expected: new release uses math.gcd.

3. Verify:

   python -c "from math import gcd; print(gcd(12, 8))"

   Expected: prints 4.

When this applies

Use this when code does from fractions import gcd on Python 3.9+.

When it does not apply

If the error names a different fractions member, check what Python 3.9 removed. If you are on Python 3.8 or older, something else is wrong.

Compatibility

Python 3.9+. math.gcd exists since 3.5, so the new import works on 3.5 through current.

Root cause

fractions.gcd was a deprecated alias of math.gcd since Python 3.5 and was finally removed in 3.9. Code that never migrated breaks at import on new Python.

Edge cases

Old networkx and snakemake releases are the most common carriers; upgrading them fixes it without code changes. When supporting old Pythons too, the try/except import pattern covers both.

GitHub issue snakemake/snakemake#1014 reports ImportError: cannot import name 'gcd' from 'fractions' on Python 3.9+. The verified answer: fractions.gcd was removed in Python 3.9 (deprecated since 3.5 in favor of math.gcd), so old code importing it breaks; the fix is changing the import to from math import gcd or upgrading the package to a release that did. Full thread: https://github.com/snakemake/snakemake/issues/1014