# Is there a duck-typing way to know we can't compile extensions?

[nedbat]

Originally reported by Zooko O'Whielacronx (Bitbucket: zooko, GitHub: zooko)

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Is there a duck-typing way to know we can't compile extensions?

Yes! You try to build the extension and if it fails then you automatically fall back to using your pure-Python alternative. This way install will also succeed on a system without a C compiler, for example.

The example code that I crib from whenever I'm going to use this hack is simplejson:

http://simplejson.googlecode.com/svn/trunk/setup.py

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• Bitbucket: https://bitbucket.org/ned/coveragepy/issue/80

[nedbat]

I'm experimenting with this code:

try:
    setup(**setup_args)
except:
    del setup_args['ext_modules']
    setup(**setup_args)

It works, but might be a bit over-zealous in catching exceptions. The simplejson code catches BuildFailed, but I find on Windows that when I can't install the extension, it fails with ValueError:

...
  File "c:\python26\Lib\distutils\msvc9compiler.py", line 458, in compile
    self.initialize()
  File "c:\python26\Lib\distutils\msvc9compiler.py", line 368, in initialize
    vc_env = query_vcvarsall(VERSION, plat_spec)
  File "c:\python26\Lib\distutils\msvc9compiler.py", line 284, in query_vcvarsall
    raise ValueError(str(list(result.keys())))
ValueError: [u'path']

[nedbat]

Oops, sorry, just read further into the simplejson code, it throws its own BuildFailed error. I'll look into using its technique.

[nedbat]

Fixed in <<changeset 740b68eb8fc5 (bb)>>.

[nedbat]

Original comment by Domen Kožar (Bitbucket: iElectric, GitHub: iElectric)

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This still fails if no gcc is present: http://dpaste.org/A1WwH/

[nedbat]

Original comment by Domen Kožar (Bitbucket: iElectric, GitHub: iElectric)

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This is under zc.buildout.

[nedbat]

Original comment by Domen Kožar (Bitbucket: iElectric, GitHub: iElectric)

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Probably needs to use https://bitbucket.org/zzzeek/sqlalchemy/src/b91c1cc4c893741443534a112b1d2d74321fce64/setup.py?at=default#cl-115

[nedbat]

Original comment by Domen Kožar (Bitbucket: iElectric, GitHub: iElectric)

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I'm afraid zc.buildout doesn't pass "install" to setup.py, taking the route sqlalchemy does would stop such issues.

[nedbat]

Domen, thanks for the extra information. I'm afraid I've never used zc.buildout, can you provide very detailed instructions on how to reproduce this? You mention zc.buildout doesn't pass "install" to setup.py, how can that be right? Maybe I don't understand what this is all meant to accomplish. I don't see how the sqlalchemy code gets around not having a verb passed to it.

[nedbat]

Original comment by Domen Kožar (Bitbucket: iElectric, GitHub: iElectric)

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Sure:

• git clone git://gist.github.com/4122350.git
• cd 4122350 (bb)
• python bootstrap.py
• bin/buildout

(make sure you don't have gcc installed)

[nedbat]

Original comment by Domen Kožar (Bitbucket: iElectric, GitHub: iElectric)

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So what SQLAlchemy does different that is more bullet-proof is the following: https://bitbucket.org/zzzeek/sqlalchemy/src/b91c1cc4c893741443534a112b1d2d74321fce64/setup.py?at=default#cl-61

[nedbat]

Original comment by Domen Kožar (Bitbucket: iElectric, GitHub: iElectric)

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Would you accept pull-request to fix this?

[nedbat]

I would accept a pull request, provided I could understand it, and it supported Py2.3 - Py3.3, and worked on Windows, Linux, and Mac. This part of the setup.py doesn't have automated tests, but some guidance on how to make sure it keeps working would be good too.

[nedbat]

Original comment by Zooko O'Whielacronx (Bitbucket: zooko, GitHub: zooko)

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One way to reduce the risks of undetected breakage on different platforms would be to hew as closely as possible to what other packages do.

Simplejson is very widely used and actively supported, so it's setup.py probably works:

https://github.com/simplejson/simplejson/blob/master/setup.py

By the way, please add PyPy to your list of platforms that you don't want to break! ☺

[nedbat]

Original comment by Domen Kožar (Bitbucket: iElectric, GitHub: iElectric)

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(Reply via do...@dev.si):

simplejson is doing the same thing: catching exception early and raising
BuildFaild that is caught later on.

On Mon, Nov 26, 2012 at 4:46 PM, Zooko O'Whielacronx <
issues-reply@bitbucket.org> wrote:

[nedbat]

Hmm, I have a complaint that 3.5.3 fails where 3.5.2 worked. Maybe simplejson's way is the way to go.

[nedbat]

Handled in the way SQLAlchemy and simplejson do (they use the same way!) in <<changeset fd9b29a3383188ae87692fdd0a030f4f2ceca858 (bb)>>.