Python Check for None True or False

General Discussions
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1

How should I check if a variable is None, True, or False in Python?

I have a function that can return one of three things:

  • success (True)
  • failure (False)
  • error reading/parsing stream (None)

Currently, I’m testing the result like this:

result = simulate(open("myfile"))
if result == None:
    print("Error parsing stream")
elif result == True: # Shouldn't do this
    print("Result pass")
else:
    print("Result fail")

Is it as simple as removing the == True part, or should I consider using a tri-state (tri-bool) data type? I do not want the simulate function to throw an exception, as all I want the outer program to do with an error is log it and continue.

Python check if variable is None: How can I improve the way I check for None, True, or False in a Pythonic way?

2

Hey, I’ve dealt with similar scenarios before. You can handle this more robustly by leveraging exception handling in Python. Here’s how:

try:
    result = simulate(open("myfile"))
except SimulationException as sim_exc:
    print("Error parsing stream:", sim_exc)
else:
    if result:
        print("Result pass")
    else:
        print("Result fail")

This approach works well because it ensures your program doesn’t break even if there’s an error. Instead of relying on tri-state checks, you’re explicitly catching exceptions, logging the issue, and continuing execution. It’s particularly useful when simulate might throw specific exceptions, making your code more descriptive and fault-tolerant.

For me, this has been a reliable way to deal with uncertain return types in functions. What do you think?

3

Adding to what Miro mentioned, I’d suggest another approach that’s simple and keeps things Pythonic. Explicitly check for None instead of relying on exceptions, like this:

result = simulate(open("myfile"))

if result is None:
    print("Error parsing stream")
elif result:
    print("Result pass")
else:
    print("Result fail")

This is cleaner and ensures you’re directly addressing the potential tri-state nature of result. Using if result is None is considered more Pythonic than comparing with None using ==. It makes your intent clear and avoids unnecessary assumptions about how truthy or falsy values should behave.

In my experience, this explicit check is great for situations where you expect a None result as part of normal operation and want to handle it differently without resorting to exceptions.

What’s your take on this approach?

4

Both Miro’s and Sam’s suggestions are excellent, but if you’re looking for a more compact, readable way to handle this logic, you can use Python’s ternary conditional operator:

result = simulate(open("myfile"))

print("Error parsing stream" if result is None else ("Result pass" if result else "Result fail"))

This one-liner is ideal for straightforward scenarios where the logic is simple and doesn’t need extensive error handling. It’s concise yet effective, making the code easier to read at a glance.

However, if the checks grow in complexity, sticking to a more verbose structure (like the ones Miro and Sam shared) might be better. Personally, I find this approach works best for quick scripts or prototyping.

Curious to hear if this resonates with your coding style!