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A Comprehensive Guide to Passing Arguments to Python Functions and Handling Return Values

Updated: at 03:23 AM

In Python programming, arguments allow you to pass data to functions. Understanding how to properly define function parameters and pass arguments is crucial for building reusable and maintainable Python programs. This guide will provide a thorough overview of passing different argument types to Python functions and handling return values.

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Arguments or parameters allow functions in Python to accept input data and perform operations on them. There are four types of arguments that can be passed to Python functions:

In addition to passing arguments, functions can also return values back to the caller. Return values allow functions to output results that can be stored or used elsewhere in your code. Properly handling return values is important for writing reusable Python functions.

In this guide, you will learn:

Let’s get started with examples of passing arguments to Python functions!

Required Arguments

Required arguments, as the name suggests, are arguments that must be provided when calling a function. They are positioned in the function definition based on the order they should be passed to the function.

For example:

# Define function with two required arguments
def full_name(first, last):
  return first + " " + last

# Call with required arguments in proper order
print(full_name("John", "Doe"))

# Output: John Doe

In this full_name() function, first and last are required parameters. When calling full_name(), valid string arguments must be passed in that order inside the parentheses. Required parameters ensure source data is available for the function to use.

Let’s look at some key tips for using required arguments:

Overall, required arguments allow functions to mandatory input data needed for them to operate properly.

Keyword Arguments

Keyword arguments provide a way to identify arguments by parameter name rather than only by position. They can be passed in any order since the Python interpreter matches them by name.

Here is an example function using keyword arguments:

# Function definition with keyword parameters
def user_info(name, age, location):
  print(f"Name: {name}")
  print(f"Age: {age}")
  print(f"Location: {location}")

# Call function using keyword arguments
user_info(age=30, location="New York", name="James")

In user_info(), the arguments are keyed by the parameter name. When calling the function, we pass age, location, and name in any order by specifying the keyword name before the value.

Some key advantages of keyword arguments:

In general, use keyword arguments when the order of parameters may not be clear or if you want to explicitly match values to names.

Default Arguments

Default arguments allow parameters to have a preset value if no argument is passed. This allows flexibility when calling the function using that parameter.

Here is an example of a default parameter:

# Function with default 'count' parameter
def print_nums(max, count=5):
  for x in range(count):

  print("Maximum: ", max)

print_nums(7) # Uses default count
print_nums(5, 2) # Overrides the default

In print_nums(), we set a default value of 5 for count. This means we can call print_nums() with just the max argument and it will use the default 5 for count. We can also override the default by passing a count value.

Some best practices when using default arguments:

Default parameters allow your functions to be flexible and prevent errors from missing arguments!

Variable-Length Arguments

Variable-length arguments allow a function to accept an arbitrary number of positional or keyword arguments. This provides dynamic flexibility when calling functions.

There are two types of variable-length arguments in Python:

Arbitrary Argument Lists

Defined using a * prefix on the parameter name. This will capture all remaining positional arguments passed to the function in a tuple.

For example:

# Accepts variable number of pos. args
def sum_all(*nums):
  total = 0

  for n in nums:
    total += n

  return total

sum_all(1, 5, 8) # nums = (1, 5, 8)

*nums will be a tuple containing each of the arbitrary positional arguments passed when calling sum_all().

Arbitrary Keyword Arguments

Defined using a ** prefix on the parameter name. This will capture any remaining keyword arguments in a dictionary.

For example:

# Accepts variable number of keyword args
def print_kwargs(**kwargs):
  for k, v in kwargs.items():
    print(f"{k}: {v}")

print_kwargs(name="John", age=25)
# Prints name: John, age: 25

Here **kwargs lets us accept any number of keyword arguments when calling the function. The kwargs parameter will be a dictionary containing the keyword argument name and value pairs.

Key tips for using variable-length arguments:

Variable-length arguments provide dynamic flexibility when you don’t know how many arguments a function may need to accept!

Data Type and Value Validation

When accepting arguments in Python, it can be important to validate the data type and values of the arguments passed to a function. This helps catch errors and invalid data early and prevent bugs.

Here are some ways to validate function arguments in Python:

Type Checking

Use the isinstance() or type() functions to check the type of arguments passed:

def multiply(num1, num2):
  if not isinstance(num1, (int, float)):
    raise TypeError("Num1 must be numeric")

  if not isinstance(num2, (int, float)):
    raise TypeError("Num2 must be numeric")

  return num1 * num2

This checks that both num1 and num2 are integers or floats before multiplying.

Value Checking

Verify argument values meet expected criteria with conditionals:

def set_volume(volume):

  if not 0 <= volume <= 10:
    raise ValueError("Volume must be between 0 and 10")

  print(f"Setting volume to {volume}")

Here we validate if volume is between 0 and 10.

Argument Data Validation

Dedicated packages like PyDantic can also validate more complex data types and structures like dictionaries.

Validating arguments is crucial for writing robust Python functions that handle bad data properly!

Returning Values

The return statement is used to exit a function and send a value back to the caller. Proper use of return values is important for making Python functions more robust and reusable.

Here is an example of returning a value from a function:

def sum_nums(nums):
  total = 0
  for n in nums:
    total += n
  return total # Return computed value

sums = sum_nums([1, 5, 8])
print(sums) # Prints 14

return total exits the function and sends back the sum to be assigned to sums in the caller.

Some key points for return values:

Properly handling return values makes your functions more flexible, reusable, and less prone to side effects!

Handling Return Values

When calling Python functions that return values, we need to properly handle the return values for downstream use. Here are some ways to handle return values:

Storing in Variables

Capture return values in variables for later access:

# Call function and store return value
results = process_data(data)

# Use return value

Returning Tuples

Functions can return tuples to handle multiple return values:

def min_max(nums):
  return min(nums), max(nums) # Return tuple

min_val, max_val = min_max([1, 2, 9, 4])

Unpacking Return Values

If you don’t need to store return values, omit the variable assignment:

# Call function and discard return value

This ignores or discards the return value.

Chaining Functions

Call functions directly on previous return values:

# Chained function calls

The return value of one function feeds into the next.

Properly handling return values via variables, tuples, chaining, and unpacking allows you to fully leverage function output in your code!


Mastering the use of arguments and return values is critical for writing clean, robust functions in Python. The key takeaways are:

Applying these best practices to your Python functions will make your code more readable, maintainable, and less error-prone. Your functions will transform into flexible and powerful building blocks for software development.