CODEWITHSUNDEEP
pythonbooleantruthiness
void.pointer
void.pointer

Reputation: 26405

How to evaluate string as boolean in return statement?

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.

Upvotes: 2

Views: 5273

Answers (5)

mjwunderlich
mjwunderlich

Reputation: 1035

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

Upvotes: 0

tkdkop
tkdkop

Reputation: 129

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)

Upvotes: 1

Martijn Pieters
Martijn Pieters

Reputation: 1123900

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.

Upvotes: 8

Douglas - 15 year old Pythoner
Douglas - 15 year old Pythoner

Reputation: 398

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.

Upvotes: -2

zangw
zangw

Reputation: 48506

>>> 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.

Upvotes: 1

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