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I have a function like this in Python 3.4:

def is_value_valid(the_str):
    return len(the_str) != 0

There are of course other ways to write this, such as return the_str != "". Are there more pythonic ways of writing this expression? I am familiar with the concepts of truthy/falsy, so I know I could shortcut this in an if condition:

if not the_str:
    # do stuff

But the if expects a boolean result in its expression (this is my naive oversimplification here as a C++ programmer; I'm not familiar with standardese for this). However, there is nothing there to force the expression to evaluate to boolean in the return statement. I have tried just returning the string, and as long as no one treats the returned value as a string, it works just fine in calling code under boolean context. But from a post-condition perspective, I don't want to return a non-boolean type.

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    bool(the_str)?? Commented Jan 12, 2016 at 14:36
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    @Burhan Why not simply if not the_str? Commented Jan 12, 2016 at 14:45
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    IMHO, the most Pythonic way would be to simply use the if statement. Why waste a function call for something so trivial? Unless, of course, this is just a simplified example. Commented Jan 12, 2016 at 14:45
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    I truly do not understand the reasoning behind explicitly casting the value, and would appreciate clarification. (I gather that there's some attempt to explain it in the question, but I don't get it.) Commented Jan 12, 2016 at 14:46
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    @PM2Ring: I have tried just returning the string, and as long as no one treats the returned value as a string, it works just fine in calling code under boolean context. But from a post-condition perspective, I don't want to return a non-boolean type.. So the intention is for the function to return a boolean. Returning a string instead exposes an implementation detail of the function and returns the wrong type. bool() is the only proper solution for this use-case. Commented Jan 12, 2016 at 15:05

5 Answers 5

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8

This is exactly what the bool() function does; return a boolean based on evaluating the truth value of the argument, just like an if statement would:

return bool(the_str)

Note that if doesn't expect a boolean value. It simply evaluates the truth value of the result of the expression.

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7 Comments

return True if the_str else False. I see the equivalent of this weird ternary idiom in PHP all the time. I just don't get why we're doing this exercise. :(
@kojiro: if a function promises to return a boolean, it is better to actually return a boolean. That makes for a cleaner interface.
Is that why? From the question, it sounds like the argument is that if expects a boolean, not that a function promises it.
@kojiro: That's the post-condition perspective; the post-condition is an expectation about what a function does. The post-condition here is that a boolean is returned.
@kojiro: There are loads of antipatterns in wide use, waiting to become bugs. Like this one or this one, see Why is Python 3.x's super() magic?. There are a lot of programmers out that that don't understand their craft.
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1 
>>> bool("foo")
True
>>> bool("")
False

Empty strings evaluate to False, but everything else evaluates to True. So this should not be used for any kind of parsing purposes.

So just return bool(the_str) in your case.

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1

The if statement automatically evaluates a string as a boolean. However, if you want to return your string as a boolean you would simply do

return bool(the_str)
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You are very late to this party. Why repeat what others have already stated 15 minutes ago?
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bool(the_str) is definitely the way to go, as several have mentioned.

But if your method requires that a string be given, I would also test for the string actually being a string (because bool(5) and bool("this is a string") will both return true.

return isinstance(the_str, str) and bool(the_str)

Or when used in an if statement:

if isinstance(the_str, str) and the_str:
    # here we are sure it's a string whose length > 0
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4 Comments

In an if context, just use isinstance(the_str, str) and the_str; using bool() there is entirely redundant.
This is not what the question is asking about though. They are asking how to produce a boolean to return from a function. It is not about validating that the value tested is a string, only how to produce a boolean for the truth value.
I would argue it depends on context -- for some methods return isinstance(the_str, str) and bool(the_str) might be what's necessary
That would return False or the value of the_str again, so not always a boolean.
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def is_value_valid(the_str):
    return bool(len(the_str) != 0)

Using bool(x) converts the value to a boolean. This function takes len(the_str) != 0, evaluates it, converts it to bool, then returns the value.

The bool(x) is not required, you can just have the parentheses, because it will already return a bool, as evaluations return boolean by default.

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1 Comment

!= already returns a bool, wrapping it again in bool() is entirely superfluous.

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