Defining a Function

Python functions are defined with the def keyword followed by the name of the function and parentheses:

def hello():
  print("Hello, world!")
hello()

In this example, the hello() function prints the given message to the console.

Here is the output:

Defining a Function with Parameters

Python functions can be defined with parameters, which help in passing data to the function:

def hello(name):
  print(f"Hello, {name}!")
hello("Robin")

In this example, a name is passed to the function as an argument, which is stored in the name parameter (variable).

Here is the output:

Note: Function names in Python are written in snake_case.

Returning Values from a Function

In Python, a value can be returned from a function in two ways:

• Using the return keyword
• Using the yield keyword

Returning Values Using return

The return keyword is used to return a value from a Python function. The value returned can then be stored in a variable and used in the program.

In this example, the check_leap_year() function returns a string that indicates if the given year is a leap year or not:

def check_leap_year(year):
  if year % 4 == 0:
    return str(year) + " is a leap year."
  else:
    return str(year) + " is not a leap year."
year_to_check = 2018
returned_value = check_leap_year(year_to_check)
print(returned_value)

The resulting output will look like this:

Returning Values Using yield

A function can also return values with the yield keyword. Like return, yield suspends the function’s execution and returns the value specified. Unlike return, the yield statement retains the state of the function and will resume where it left off on the next function call (i.e., execution resumes after the last yield statement). This way, the function can produce a number of values over time.

Functions using yield rather than return are known as generator functions. Such a function can be used as an iterator.

This example will automatically generate successive Fibonacci numbers:

def fibonacci():
  a = 1
  yield a
  b = 1
  yield b
  while True:
    c = a + b
    yield c
    a = b
    b = c
for num in fibonacci():
  if num > 50:
    break
  print(num)

Here is the output:

Python Library Functions

Python library functions are built-in functions that are always available for use without needing to define them explicitly. These functions are a part of the Python standard library and help perform common tasks like mathematical operations, type conversions, string manipulations, and more.

Some examples for Python library functions are:

• print()
• max()
• min()
• sum()
• abs()
• type()

Example: Using the len() Function

The len() function is a commonly used library function in Python that returns the total number of elements in an object like a string, list, tuple, etc.

my_list = [10, 20, 30, 40]
print(len(my_list))

Here is the output:

Higher-Order Functions

In Python, functions are treated as first-class objects. This means that they can be assigned to variables, stored in data structures, and passed to or returned from other functions.

Functions are considered to be higher-order values because they can be used as parameters or return values for other functions.

Example: Using the filter() Function

The built-in filter() function in Python is used to filter elements from an iterable (like lists, tuples, or sets) that satisfy a specific condition:

def is_perfect_square(n):
  return (n ** 0.5).is_integer()
numbers = [3, 4, 37, 9, 7, 32, 25, 81, 79, 100]
perfect_squares = filter(is_perfect_square, numbers)
print(list(perfect_squares))

filter() takes a predicate (a function that returns a boolean value) and an iterable, and returns a new iterable containing all elements of the first one that makes the predicate True.

Here is the output:

Codebyte Example: Calculate the Area of a Rectangle

This codebyte example defines a function to calculate the area of a rectangle with the help of its length and width:

Frequently Asked Questions

1. What’s the difference between a parameter and an argument in a Python function?

• A parameter is a variable used in the function definition.
• An argument is the value passed to the function when it’s called.

2. What happens if you call a function before defining it?

Python will raise a NameError because the function must be defined before it is called.

3. What is a default parameter in a Python function?

A default parameter in a Python function is a value that is used if no argument is provided:

def hello(name="Guest"):
  print(f"Hello, {name}!")

Here, the given value will be used for the name parameter (variable) if no argument is passed to the function.