Functions

In addition to operators like + and -, Python also includes many functions. Functions can be used by calling them.

We've actually already seen a function: the print function.

We used print by calling it like this:

>>> name = "Trey"
>>> print("Hello", name)
Hello Trey

You can call a function by putting parentheses after it.

See Calling a function in Python for more.

Built-in Functions

Python comes with dozens of built-in functions.

The int function can convert a string that represents an integer into an actual integer:

>>> number = "5"
>>> number
'5'
>>> int(number)
5

The str function can convert any object to a string:

>>> str(5)
'5'

The type function can give us the type of an object:

>>> type("5")
<class 'str'>
>>> type(5)
<class 'int'>
>>> type(5.0)
<class 'float'>

The dir and vars functions can be helpful for debugging your code, especially while playing around in the Python REPL. They're ways to ask "what variables are defined right now and what are their values"?

>>> dir()
['__annotations__', '__builtins__', '__doc__', '__loader__', '__name__', '__package__', '__spec__', 'name', 'number']
>>> vars()
{'__name__': '__main__', '__doc__': None, '__package__': None, '__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__spec__': None, '__annotations__': {}, '__builtins__': <module 'builtins' (built-in)>, 'name': 'Trey', 'number': '5'}

You can ignore the variables with double underscores around them. Those are special variables that mean something special to Python and we don't usually define those ourselves.

Other Functions

You can also find functions in the Python standard library (used by importing a module) as well and you can even define your own functions. We'll see how to do that later.

Methods

Many types of Python objects also include functions that live on the object itself. These are called methods.

For example strings have a lower method, which returns a lowercase version of the string:

>>> name = "Trey"
>>> name.lower()
'name'

You can refer to a method by putting a period (.) after a reference to the object and then writing the method name. Methods can be called just as functions can be.

Want to play with some methods? Try out some of Python's string methods on a new string!

Breakpoint for debugging

There's one function you'll never use in your working Python code, but which can be very handy when you're troubleshooting a broken Python program: the breakpoint function.

The breakpoint function allows us to drop into the "Python debugger", which is similar to the Python REPL, though it's slightly different.

The Python debugger has a number of commands you can lookup on your own, but these commands are likely the most useful:

n (or next): Run the next line of code
l` (or ``list): List the surrounding code lines
interact Starts an interactive Python interpreter (REPL)
c (or continue): Continue running (until a breakpoint or exit)

Try using breakpoint to debug this Python program:

from random import randint

answer = randint(0, 1)
n = input("Guess: 0 or 1? ")
if n == answer:
    print("Correct!")
else:
    print(f"Incorrect. The answer was {answer}.")

When we run this program it always says our guess was incorrect, even when it was correct!

$ python3 guess.py
Guess: 0 or 1? 0
Incorrect. The answer was 0.

Let's put a call to breakpoint just after that ``answer = `` line:

from random import randint

answer = randint(0, 1)
breakpoint()
n = input("Guess: 0 or 1? ")
if n == answer:
    print("Correct!")
else:
    print(f"Incorrect. The answer was {answer}.")

Using breakpoint is a valuable skill for debugging Python code. So is reading a traceback in Python.