Python dictionary size/entry limit?

Stefan Behnel stefan_ml at
Sat Feb 21 11:47:10 CET 2009

intelliminer at wrote:
> I wrote a script to process textual data and extract phrases from
> them, storing these phrases in a dictionary. It encounters a
> MemoryError when there are about 11.18M keys in the dictionary, and
> the size is about 1.5GB.
> [...]
> I have 1GB of pysical memory and 3GB in pagefile. Is there a limit to
> the size or number of entries that a single dictionary can possess? By
> searching on the web I can't find a clue why this problem occurs.

Python dicts are only limited by what your OS returns as free memory.
However, when a dict grows, it needs to resize, which means that it has to
create a bigger copy of itself and redistribute the keys. For a dict that
is already 1.5GB big, this can temporarily eat a lot more memory than you
have, at least more than two times as much as the size of the dict itself.

You may be better served with one of the dbm databases that come with
Python. They live on-disk but do the usual in-memory caching. They'll
likely perform a lot better than your OS level swap file.


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