Type Hinting

In this lesson, you’ll learn about type hinting in Python. Type hinting is a formal solution to statically indicate the type of a value within your Python code. It was specified in PEP 484 and introduced in Python 3.5.

Here’s an example of adding type information to a function. You annotate the arguments and the return value:

def greet(name: str) -> str:
    return "Hello, " + name

The name: str syntax indicates the name argument should be of type str. The -> syntax indicates the greet() function will return a string.

The following example function turns a text string into a headline by adding proper capitalization and a decorative line:

>>> def headline(text, align=True):
...    if align:
...        return f"{text.title()}\n{'-' * len(text)}"
...    else:
...        return f" {text.title()} ".center(50, "o")
...
...
>>> print(headline("python type checking"))
Python Type Checking
--------------------

>>> print(headline("python type checking", align=False))
oooooooooooooo Python Type Checking oooooooooooooo

Now add type hints by annotating the arguments and the return value as follows:

>>> def headline(text: str, align: bool = True) -> str:
...    if align:
...        return f"{text.title()}\n{'-' * len(text)}"
...    else:
...        return f" {text.title()} ".center(50, "o")
...
...
>>> headline
<function headline at 0x105b81268>

>>> print(headline("python type checking", align="left"))
Python Type Checking
--------------------

>>> print(headline("python type checking", align="center"))
Python Type Checking
--------------------

In terms of style, PEP 8 recommends the following:

• Use normal rules for colons, that is, no space before and one space after a colon (text: str).
• Use spaces around the = sign when combining an argument annotation with a default value (align: bool = True).
• Use spaces around the -> arrow (def headline(...) -> str).

To learn more about f-strings, check out the following resources:

• Python 3’s f-Strings: An Improved String Formatting Syntax (Guide)
• Python 3’s f-Strings: An Improved String Formatting Syntax