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

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I've been hearing a lot about the
PyPy
project. They claim it is 6.3 times faster than the href="http://en.wikipedia.org/wiki/CPython" rel="noreferrer">CPython
interpreter on their
site.

Whenever we talk about dynamic
languages like Python, speed is one of the top issues. To solve this, they say PyPy is
6.3 times faster.

The second issue is
parallelism, the infamous href="http://en.wikipedia.org/wiki/Global_Interpreter_Lock" rel="noreferrer">Global
Interpreter Lock (GIL). For this, PyPy says it href="http://pypy.org/tmdonate.html" rel="noreferrer">can give GIL-less
Python.

If PyPy can solve these great
challenges, what are its weaknesses that are preventing wider adoption? That is to say,
what's preventing someone like me, a typical Python developer, from switching to PyPy
right now?

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class="normal">Answer

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. Cython and Numpy are awesome
   for numerics, and most people who actually need speed in Python are using those (+
   Pandas, SciPy, etc.) heavily. Since they're either non-existent or tenuously supported
   and slow the people who need a fast Python often are better off with
   CPython both for speed and
   ease-of-use.


2. Python 3 support
   is experimental at the moment. has just
   reached stable! As of 20th June 2014, href="http://morepypy.blogspot.co.uk/2014/06/pypy3-231-fulcrum.html"
   rel="noreferrer">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.

NOTE: This question is ancient! Avoid drawing
conclusions from out-of-date
information.