what-is-true.md

What is true?

Now we understand how code like this works.

message = input("Enter something: ")
if message == '':
    print("You didn't enter anything!")
else:
    print("You entered:", message)

But most Python programmers would write that code like this instead:

message = input("Enter something: ")
if message:
    print("You entered:", message)
else:
    print("You didn't enter anything!")

What the heck was that? We did if message, but message was a string, not True or False!

Python converted our message to a Boolean and then checked if the Boolean it ended up with was True. But when will it be true?

Converting to Booleans

The if message: actually did the same thing as if bool(message), which is same as if bool(message) == True:. Usually we just don't write the ==True part anywhere because we don't need it.

We can convert things to Booleans like Python did by doing bool(things). Let's try that with strings.

>>> bool('hello')
True
>>> bool('there')
True
>>> bool('True')
True
>>> bool('False')    # this isn't special in any way
True
>>>

As we can see, the Boolean value of most strings is True. The only string that has a false Boolean value is the empty string, '' or "":

>>> bool('')
False
>>>

Most other things are also treated as False if they're empty and True if they're not empty.

>>> bool([1, 2, 3])
True
>>> bool([])
False
>>> bool((1, 2, 3))
True
>>> bool(())
False
>>> bool({'a': 1, 'b': 2})
True
>>> bool({})
False
>>>

None and zero are also falsy, but positive and negative numbers are treated as True.

>>> bool(None)
False
>>> bool(0)
False
>>> bool(0.0)
False
>>> bool(1)
True
>>> bool(-1)
True
>>>

Most other things are also treated as True.

>>> bool(OSError)
True
>>> bool(print)
True
>>>

When and why should we use Boolean values of things?

It's recommended to rely on the Boolean value when we're doing something with things like lists and tuples. This way our code will work even if it gets a value of a different type than we expected it to get originally.

For example, this code doesn't work right if we give it something else than a list. It thinks that empty tuples, strings and dictionaries aren't empty just because they aren't empty lists:

>>> def is_this_empty(thing):
...     if thing == []:
...         print("It's empty!")
...     else:
...         print("It's not empty.")
...
>>> is_this_empty([1, 2, 3])
It's not empty.
>>> is_this_empty([])
It's empty!
>>> is_this_empty(())
It's not empty.
>>> is_this_empty('')
It's not empty.
>>> is_this_empty({})
It's not empty.
>>>

We could improve the code by checking against different empty things.

>>> def is_this_empty(thing):
...     if thing == [] or thing == () or thing == '' or thing == {}:
...         print("It's empty!")
...     else:
...         print("It's not empty.")
...
>>>

But Python has many other data types that can be empty and we haven't talked about in this tutorial. Trying to check all of them would be pointless because functions like this already work with all of them:

>>> def is_this_empty(thing):
...     if thing:
...         print("It's not empty.")
...     else:
...         print("It's empty!")
...
>>>

There's also cases when we should not rely on the Boolean value. When we're doing things with numbers and None it's best to simply compare to None or zero. Like this:

if number != 0:
    print("number is not zero")

if value is not None:
    print("value is not None")

Not like this:

if number:
    print("number is not zero")

if value:
    print("value is not None")

We used is not instead of != in the first example because the official style guide recommends it. The reason is that it's possible to create a value that isn't really None but seems like None when we compare it with None using == or !=, and we want to make sure that we don't treat values like that as None.

So here's how we should check if something is None:

if not value: ...      # not good if we want to check if it's None
if value == None: ...  # better
if value is None: ...  # best

Summary

• if thing: does the same thing as if bool(thing):. This also works with while loops and most other things that are usually used with Booleans.
• bool() of most things is True, but bool() values of None, zero and most empty things are False.
• Use is and is not when comparing to None, == and != when checking if a number is zero and rely on the Boolean value when checking if something is empty.

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You may use this tutorial freely at your own risk. See LICENSE.

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