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 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 Global Interpreter Lock  GIL   For this  PyPy says it 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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Q  If PyPy can solve these great challenges  speed  memory consumption  parallelism  in comparison to CPython  what are its weaknesses that are preventing wider adoption   A  First  there is little evidence that the PyPy team can solve the speed problem in general  Long-term evidence is showing that PyPy runs certain Python codes slower than CPython and this drawback seems to be rooted very deeply in PyPy   Secondly  the current version of PyPy consumes much more memory than CPython in a rather large set of cases  So PyPy didn t solve the memory consumption problem yet   Whether PyPy solves the mentioned great challenges and will in general be faster  less memory hungry  and more friendly to parallelism than CPython is an open question that cannot be solved in the short term  Some people are betting that PyPy will never be able to offer a general solution enabling it to dominate CPython 2 7 and 3 3 in all cases   If PyPy succeeds to be better than CPython in general  which is questionable  the main weakness affecting its wider adoption will be its compatibility with CPython  There also exist issues such as the fact that CPython runs on a wider range of CPUs and OSes  but these issues are much less important compared to PyPy s performance and CPython-compatibility goals     Q  Why can t I do drop in replacement of CPython with PyPy now   A  PyPy isn t 100  compatible with CPython because it isn t simulating CPython under the hood  Some programs may still depend on CPython s unique features that are absent in PyPy such as C bindings  C implementations of Python object amp methods  or the incremental nature of CPython s garbage collector

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The second question is easier to answer  you basically can use PyPy as a drop-in replacement if all your code is pure Python   However  many widely used libraries  including some of the standard library  are written in C and compiled as Python extensions   Some of these can be made to work with PyPy  some can t   PyPy provides the same  forward-facing  tool as Python --- that is  it is Python --- but its innards are different  so tools that interface with those innards won t work   As for the first question  I imagine it is sort of a Catch-22 with the first  PyPy has been evolving rapidly in an effort to improve speed and enhance interoperability with other code   This has made it more experimental than official   I think it s possible that if PyPy gets into a stable state  it may start getting more widely used   I also think it would be great for Python to move away from its C underpinnings   But it won t happen for a while   PyPy hasn t yet reached the critical mass where it is almost useful enough on its own to do everything you d want  which would motivate people to fill in the gaps

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Because pypy is not 100  compatible  takes 8 gigs of ram to compile  is a moving target  and highly experimental  where cpython is stable  the default target for module builders for 2 decades  including c extensions that don t work on pypy   and already widely deployed   Pypy will likely never be the reference implementation  but it is a good tool to have

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I ve found examples  where PyPy is slower than Python  But  Only on Windows   C  Users User gt python -m timeit -n10 -s from sympy import isprime   isprime 2  521-1  isprime 2  1279-1   10 loops  best of 3  294 msec per loop  C  Users User gt pypy -m timeit -n10 -s from sympy import isprime   isprime 2  521-1  isprime 2  1279-1   10 loops  best of 3  1 33 sec per loop   So  if you think of PyPy  forget Windows  On Linux  you can achieve awesome accelerations  Example  list all primes between 1 and 1 000 000     from sympy import sieve primes   list sieve primerange 1  10  6     This runs 10    times faster on PyPy than on Python  But not on windows  There it is only 3x as fast

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For a lot of projects  there is actually 0  difference between the different pythons in terms of speed  That is those that are dominated by engineering time and where all pythons have the same amount of library support

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PyPy has had Python 3 support for a while  but according to this HackerNoon post by Anthony Shaw from April 2nd  2018  PyPy3 is still several times slower than PyPy  Python 2     For many scientific calculations  particularly matrix calculations  numpy is a better choice  see FAQ  Should I install numpy or numpypy     Pypy does not support gmpy2  You can instead make use of gmpy cffi  though I haven t tested its speed and the project had one release in 2014    For Project Euler problems  I make frequent use of PyPy  and for simple numerical calculations often from   future   import division is sufficient for my purposes  but Python 3 support is still being worked on as of 2018  with your best bet being on 64-bit Linux  Windows PyPy3 5 v6 0  the latest as of December 2018  is in beta

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I did a small benchmark on this topic  While many of the other posters have made good points about compatibility  my experience has been that PyPy isn t that much faster for just moving around bits  For many uses of Python  it really only exists to translate bits between two or more services  For example  not many web applications are performing CPU intensive analysis of datasets  Instead  they take some bytes from a client  store them in some sort of database  and later return them to other clients  Sometimes the format of the data is changed   The BDFL and the CPython developers are a remarkably intelligent group of people and have a managed to help CPython perform excellent in such a scenario  Here s a shameless blog plug  http   www hydrogen18 com blog unpickling-buffers html   I m using Stackless  which is derived from CPython and retains the full C module interface  I didn t find any advantage to using PyPy in that case

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That site does not claim PyPy is 6 3 times faster than CPython  To quote      The geometric average of all benchmarks is 0 16 or 6 3 times faster than CPython   This is a very different statement to the blanket statement you made  and when you understand the difference  you ll understand at least one set of reasons why you can t just say  use PyPy   It might sound like I m nit-picking  but understanding why these two statements are totally different is vital   To break that down    The statement they make only applies to the benchmarks they ve used  It says absolutely nothing about your program  unless your program is exactly the same as one of their benchmarks   The statement is about an average of a group of benchmarks  There is no claim that running PyPy will give a 6 3 times improvement even for the programs they have tested  There is no claim that PyPy will even run all the programs that CPython runs at all  let alone faster

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Supported Python Versions To cite the Zen of Python   Readability counts   For example  Python 3 7 introduced dataclasses and Python 3 8 introduced fstring    There might be other features in Python 3 7 and Python 3 8 which are more important to you  The point is that PyPy does not support Python 3 7 or Python 3 8 at the moment  Shameless self-advertisement  Killer Features by Python version - if you want to know more things you miss by using older Python versions

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To make this simple  PyPy provides the speed that s lacked by CPython but sacrifices its compatibility  Most people  however  choose Python for its flexibility and its  battery-included  feature  high compatibility   not for its speed  it s still preferred though

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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     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  Some extensions are still not supported  Pandas  SciPy  etc    take a look at the list of supported packages before making the change  Note that many packages marked unsupported on the list are now supported  Python 3 support is experimental at the moment  has just reached stable  As of 20th June 2014  PyPy3 2 3 1 - Fulcrum is out  PyPy sometimes isn t actually faster for  quot scripts quot   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   PyPy s pre-JIT speeds can be bad compared to CPython  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

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CPython has reference counting and garbage collection  PyPy has garbage collection only   So objects tend to be deleted earlier and   del   is called in a more predictable way in CPython  Some software relies on this behavior  thus they are not ready for migrating to PyPy   Some other software works with both  but uses less memory with CPython  because unused objects are freed earlier   I don t have any measurements to indicate how significant this is and what other implementation details affect the memory use

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

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