Why shouldn't I use PyPy over CPython if PyPy is 6.3 times faster?

Question

I\'ve been hearing a lot about the PyPy project. They claim it is 6.3 times faster than the CPython interpreter on their site.

Whenever we talk about dynamic languag

Answer 1

NOTE: PyPy is more mature and better supported now than it was in 2013, when this question was asked. Avoid drawing conclusions from out-of-date information.

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1. PyPy, as others have been quick to mention, has tenuous support for C extensions. It has support, but typically at slower-than-Python speeds and it's iffy at best. Hence a lot of modules simply require CPython. PyPy doesn't support numpy PyPy now supports numpy. Some extensions are still not supported (Pandas, SciPy, etc.), take a look at the list of supported packages before making the change.
2. Python 3 support is experimental at the moment. has just reached stable! As of 20th June 2014, PyPy3 2.3.1 - Fulcrum is out!
3. PyPy sometimes isn't actually faster for "scripts", which a lot of people use Python for. These are the short-running programs that do something simple and small. Because PyPy is a JIT compiler its main advantages come from long run times and simple types (such as numbers). Frankly, PyPy's pre-JIT speeds are pretty bad compared to CPython.
4. Inertia. Moving to PyPy often requires retooling, which for some people and organizations is simply too much work.

Those are the main reasons that affect me, I'd say.