What's the Pythonic way to write conditional statements based on installed modules?

Question

Coming from a C++ world I got used to write conditional compilation based on flags that are determined at compilation time with tools like CMake and the like. I wonder what's the most Pythonic way to mimic this functionality. For instance, this is what I currently set depending on whether a module is found or not:

import imp

try:
    imp.find_module('petsc4py')
    HAVE_PETSC=True
except ImportError:
    HAVE_PETSC=False

Then I can use HAVE_PETSC throughout the rest of my Python code. This works, but I wonder if it's the right way to do it in Python.

Why use imp instead of just import petsc4py and catching the exception from that if it is not found? — khelwood
– khelwood, Commented Aug 5, 2019 at 15:04
In my experience, "Pythonism" usually prefers "Better to beg forgiveness than ask permission" instead of "Look before you leap", so as @khelwood said, you would use a try-except. Though if you don't actually need to use petsc4py, maybe your code is Pythonic, idk. — wjandrea
– wjandrea, Commented Aug 5, 2019 at 15:10
Possible duplicate of Conditional module importing in Python — Sayse
– Sayse, Commented Aug 5, 2019 at 15:10

Timothy Baldwin · Accepted Answer · 2019-08-05 17:30:48Z

7

Yes, it is ok. You can even issue an import directly, and use the modulename itself as the flag - like in:

try:
    import petsc4py
except ImportError
    petsc4py = None

And before any use, just test for the truthfulness of petsc4py itself.

Actually, checking if it exists, and only then trying to import it, feels unpythonic due to the redundancy, as both actions trigger an ImportError all the same. But having a HAVE_PETSC variable for the checkings is ok - it can be created after the try/except above with HAVE_PETSC = bool(petsc4py)

edited Aug 5, 2019 at 17:30

Timothy Baldwin

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answered Aug 5, 2019 at 15:13

jsbueno

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4 Comments

aaragon Over a year ago

Thanks for the answer. One last thing: I adopted the C++ way to name this type of variable (HAVE_PETSC), but perhaps in Python there's a different convention?

jsbueno Over a year ago

no, I think all caps with underlines is ok for variables that are supposed to not change across execution.

user11877195 Over a year ago

"Actually, checking if it exists, and only then trying to import it, feels unpythonic due to the redundancy" Isn't that the same for checking if module exists before ever using it? IMO it would be less redundant to divide and load your own code based on whether some dependency is installed, or better yet, fail with error "This feature is not accessible without X module, use pip install X==someversionifrequired to install it", where feature would be it's own module that would throw such exception when being loaded.

jsbueno Over a year ago

Without the try/except step, your code don't run at all - not even for being able to print that a feature won't be available or print tips on how to install the dependencies. If it is a full, non optional dependency, then yes, the code should just assume it is installed, and let the error propagate if it is not (and let the installer deal with the dependency)

Green Cloak Guy · Accepted Answer · 2019-08-05 15:10:22Z

0

The way you're doing it is more-or-less fine. In fact, The python standard library uses a similar paradigm of "try to import something and if it's not valid for some reason then set a variable somehow" in multiple places. Checking if a boolean is set later in the program is going to be faster than doing a separate try/except block every single time.

In your case it would probably just be better to do this, though:

try:
    import petsc4py
    HAVE_PETSC = True
except ImportError:
    HAVE_PETSC = False

What you have works on a paradigm level, but there's no real reason to go through importlib in this case (and you probably shouldn't use imp anyway, as it's deprecated in recent versions of python).

answered Aug 5, 2019 at 15:10

Green Cloak Guy

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