if statement - Python - import in if

Question

I wrote little wrapper for urllib (python3). Is it proper and safe to import module in if?

if self.response_encoding == 'gzip':
    import gzip

I didn't find any PEP about this code. However, it bothers me.

Answer 1

The Python standard library uses it, so it is most definitely proper and safe. See the os module source for an excellent example:

if 'posix' in _names:
    name = 'posix'
    linesep = '
'
    from posix import *
    try:
        from posix import _exit
    except ImportError:
      pass
    import posixpath as path
    import posix
    __all__.extend(_get_exports_list(posix))
    del posix

It's quite common to conditionally import modules in python. Instead of if, you'll often see a try:/except ImportError: combination too:

try:
    from subprocess import check_output
except ImportError:
    # Python 2.6 and before
    def check_output(*popenargs, **kwargs):
        from subprocess import Popen
        if 'stdout' in kwargs:
            raise ValueError('stdout argument not allowed, it will be '
                             'overridden.')
        process = Popen(stdout=PIPE, *popenargs, **kwargs)
        output, unused_err = process.communicate()
        retcode = process.poll()
        if retcode:
            cmd = kwargs.get("args")
            if cmd is None:
                cmd = popenargs[0]
            raise CalledProcessError(retcode, cmd)
        return output

Here, we basically use the moral equivalent of an if test: If you can import check_output, do so, otherwise define the full function here.

An import statement is just a re-binding of an external piece of code to a local name. Using an if control flow to control the import is no different from assigning a variable in an if statement in that regard. You need to make sure you don't end up using the name without it being defined either way.