Truthy, Falsy, and bool | Pydon't 🐍

programming python

All Python objects can be used in expressions that should return a boolean value, like in an if or while statement. Python's built-in objects are usually Falsy (interpreted as False) when they are “empty” or have “no value” and otherwise they are Truthy (interpreted as True). You can define this behaviour explicitly for your own objects if you define the __bool__ dunder method.

A Python code snippet comparing some if statements.

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“Truthy” and “Falsy”

Quoting the Python documentation,

“Any object can be tested for truth value, for use in an if or while condition or as operand of the Boolean operations below [or, and and not].”

What does that mean? It means that we can use any Python object we want whenever a boolean value is expected. Boolean values (True and False) are used in conditions, which pop up in if statements and while statements, as well as in expressions that make use of the Boolean operators or, and and not.

As a very basic example, consider this Python session:

>>> if True:
...     print("Hello, World!")
...
Hello, World!
>>> if False:
...     print("Go away!")
...
>>>

This piece of code should not surprise you, as it is very standard Python code: there are a couple of if statements that make use of explicit Boolean values. The next step is using an expression that evaluates to a Boolean value:

>>> 5 > 3
True
>>> if 5 > 3:
...     print("Hello, World!")
... 
Hello, World!

The next step is using an object that is not a Boolean value, which is what this blog post is all about:

>>> l = [1, 2, 3]
>>> if l:
...     print(l)
... 
[1, 2, 3]

This is the part that could be surprising if you have never encountered it. The reason this if statement is getting executed is because the list [1, 2, 3] is Truthy, that is, the list [1, 2, 3] can be interpreted as True in a Boolean context. How can you know if an object is “Truthy” or “Falsy”? The simplest way is to use the built-in bool function that converts any Python object to a Boolean:

>>> bool(l)
True

The way this works is really simple! There are a couple of rules that specify how this works, but these simple rules can even be simplified further with a simpler heuristic:

“A value of a given type if Falsy when it is “empty” or “without any useful value”.”

Examples of built-in types and their Falsy values include the empty list, empty set, empty tuple, empty dictionary, the number 0, None and the empty string. For example:

>>> bool([])
False
>>> bool("")
False

Of course, "without any useful value" definitely depends on what you intend to do with the value you have, so I should really specify the objective rules:

  • By default, a value is Truthy (that is, is interpreted as True).
  • An object has a Falsy value (that is, is interpreted as False) if calling len on it returns 0.

Notice that the previous rule tells us that, in general, types that are containers or sequences (types of objects for which it generally makes sense to use len on), are considered Falsy when they are empty, i.e., when they have length equal to zero. But there is one more case that gives a Falsy value:

The __bool__ dunder method

  • An object has a Falsy value (that is, is interpreted as False) if it defines a __bool__ method that returns False.

__bool__ is a dunder method (dunder stands for double underscore) that you can use to tell your objects if they are Truthy or Falsy in Boolean contexts, by implementing it in your own classes. (You have seen other dunder methods already.)

If you are not acquainted with Python's dunder methods, you may want to subscribe to the Pydon't newsletter, I will write more about them later. Until then, you may want to have a look at the Python 3 Docs and what they say about the data model.

Here is a simple example showing an object that is always taken to be Truthy:

>>> class A:  
...     pass
... 
>>> a = A()
>>> if a:
...     print("Hello, World!")
... 
Hello, World!

On the opposite end, we can consider a class whose objects will always be taken to be Falsy:

>>> class A:
...     def __bool__(self):
...             return False
...
>>> a = A()
>>> if a:
...     print("Go away!")
...

In general, your use case may be such that your object sometimes is Truthy and sometimes is Falsy.

Finally, it is very important to state the order in which the rules apply!

When given an arbitrary Python object that needs to be tested for a truth value, Python first tries to call bool on it, in an attempt to use its __bool__ dunder method. If the object does not implement a __bool__ method, then Python tries to call len on it. Finally, if that also fails, Python defaults to giving a Truthy value to the object.

Remarks

Now a couple of remarks about the functioning of Truthy and Falsy values.

A note about containers with falsy objects

We said that things like the empty list, zero, and the empty dictionary are Falsy. However, things like a list that only contains zeroes or a dictionary composed of zeroes and empty lists are not Falsy, because the containers themselves are no longer empty:

>>> bool([])
False
>>> bool({})
False
>>> bool(0)
False
>>> bool([0, 0, 0]) # A list with zeroes is not an empty list.
True
>>> bool({0: []})   # A dict with a 0 key is not an empty dict.
True

A note about checking for None

As mentioned above, None is Falsy:

>>> bool(None)
False
>>> if None:
...     print("Go away!")
...

This seems about right, as None is the go-to value to be returned by a function when the function does nothing.

Imagine someone implemented the following function to return the integer square root of a number, returning None for negative inputs (because negative numbers do not have a square root in the usual sense):

import math
def int_square_root(n):
    if n < 0:
        return None
    return math.floor(math.sqrt(n))

When you use the function above you know it returns None if the computation fails, so now you might be tempted to use your newfound knowledge about the Falsy value of None, and you might write something like the following, to check if the computation succeeded:

n = int(input("Compute the integer square root of what? >> "))
int_sqrt = int_square_root(n)
if not int_sqrt:
    print("Negative numbers do not have an integer square root.")

Now, what happens if n is 0 or 0.5?

>>> n = 0.5
>>> int_sqrt = int_square_root(n)
>>> if not int_sqrt:
...     print("Negative numbers do not have an integer square root.")
... 
Negative numbers do not have an integer square root

Which is clearly wrong, because n = 0.5 is certainly positive. Let us inspect int_sqrt:

>>> int_sqrt
0

The problem is that int_square_root returned a meaningful value (that is, it did not return None) but that meaningful value is still Falsy. When you want to check if a function returned None or not, do not rely on the Truthy/Falsy value of the return value. Instead, check explicitly if the return value is None or not:

# Use                       # Avoid
if returned is None:        # if not returned:
    # ...                   #     # ...
if returned is not None:    # if returned:
    # ...                   #     # ...

This recommendation is to avoid problems like the one outlined above.

Examples in code

Now I will show you some examples of places where using the Truthy and Falsy values of Python objects allows you to write more Pythonic code.

2D point

Let us implement a simple class to represent points in a 2D plane, which could be an image, a plot or something else. Retrieving what we already had in the article about __str__ and __repr__, we can add a __bool__ method so that the origin (the point Point2D(0, 0)) is Falsy and all other points are Truthy:

# From https://mathspp.com/blog/pydonts/pydont-confuse-str-and-repr
class Point2D:
    """A class to represent points in a 2D space."""

    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __str__(self):
        """Provide a good-looking representation of the object."""
        return f"({self.x}, {self.y})"

    def __repr__(self):
        """Provide an unambiguous way of rebuilding this object."""
        return f"Point2D({repr(self.x)}, {repr(self.y)})"

    def __bool__(self):
        """The origin is Falsy and all other points are Truthy."""
        return self.x or self.y

print(bool(Point2D(0, 1))) # True
print(bool(Point2D(0, 0))) # False
print(bool(Point2D(1, 0))) # True
print(bool(Point2D(4, 2))) # True

Notice how we defined the Truthy/Falsy value of the Point2D in terms of the Truthy/Falsy values of its components! We want the Point2D to be Falsy when self.x is 0 and self.y is also 0, which means a Point2D is Truthy if any of self.x or self.y are Truthy (that is, different from 0)!

Handling error codes or error messages

It is quite common for functions to return “error codes”: integers that encode specific things that did not go quite right, or for such functions to return error messages as strings when things don't go right. These error codes are usually such that returning 0 means everything went ok, while different other integers can mean all sorts of problems.

If you are calling such a function, you can use the Truthy value of strings and/or integers to check if something went wrong, and to handle it accordingly.

As a generic example, this is the pattern we are looking for:

return_value, error_code = some_nice_function()
if error_code:
    # Something went wrong, act accordingly.

# Alternatively, something like:
return_value, error_msg = some_other_nice_function()
if error_msg:
    print(error_msg)
    # Something went wrong, act accordingly.

Processing data

It is also very common to use Truthy and Falsy values to measure if there is still data to be processed.

For example, when I talked about the walrus operator :=, we saw a while loop vaguely similar to this one:

input_lines = []
while (s := input()):
    input_lines.append(s)
# No more lines to read.
print(len(input_lines))

This while loop essentially reads input lines while there are lines to be read. As soon as the user inputs an empty line "", the loop stops and we print the number of lines we read:

>>> input_lines = []
>>> while (s := input()):
...     input_lines.append(s)
... 
Line 1
Line 2

>>> print(len(input_lines))
2

Another common pattern is when you have a list that contains some data that you have to process, and such that the list itself gets modified as you process the data.

Consider the following example:

import pathlib

def print_file_sizes(dir):
    """Print file sizes in a directory, recurse into subdirs."""

    paths_to_process = [dir]
    while paths_to_process:
        path, *paths_to_process = paths_to_process
        path_obj = pathlib.Path(path)
        if path_obj.is_file():
            print(path, path_obj.stat().st_size)
        else:
            paths_to_process += path_obj.glob("*")

This is not necessarily the way to go about doing this, but notice the while statement, and then the if: ... else: ... block that either prints something, or extends the paths_to_process list.

Conclusion

  • Python's Truthy and Falsy values allow you to rewrite common conditions in a way that is more readable and, therefore, Pythonic.
  • You can implement your own Truthy and Falsy values in custom classes by implementing the __bool__ dunder method.
  • You should also be careful when checking if a given variable is None or not, and avoid using the Falsy value of None in those particular cases.

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References

Online references last consulted on the 9th of February of 2021.

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