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Using Function Parameters to Pass Data into Functions in Python

Updated: at 04:34 AM

In Python, parameters allow functions to accept input data and perform tasks using that data. Understanding how to use parameters is key for writing reusable and modular Python code. This comprehensive guide will explain what function parameters are, why they are useful, and provide actionable techniques for passing data into functions using parameters in Python.

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Parameters, also known as arguments, are values that are passed into a function when calling it. They allow a function to accept input data and perform its logic based on that input. Parameters make functions more flexible, reusable across different contexts, and modular.

Some key benefits of using function parameters in Python include:

This guide will cover the following topics related to passing data with function parameters in Python:

By the end, you will have a comprehensive understanding of how to leverage parameters to write cleaner, more extensible, and more Pythonic code.

Parameter Basics

Declaring and Passing Parameters

Parameters are declared inside the parentheses of the def statement when creating a function:

def function_name(param1, param2):
  # function body

When calling the function, values must be passed in for each parameter:

function_name(value1, value2)

Parameters passed to functions are also commonly referred to as arguments.

Accessing Parameter Values

Inside the function body, the parameter names can be used just like regular variables:

def double(num):
  result = num * 2
  return result

print(double(5)) # 10

Parameters can be used in expressions, assigned to new variables, printed, or manipulated in any way required by the function logic.

Positional vs Keyword Parameters

In Python, parameters can be specified positionally or explicitly by keyword.

Positional Parameters

When calling a function, positional arguments must match the order of parameters in the function definition:

def full_name(first, last):
  return first + " " + last

full_name("John", "Doe") # Valid
full_name("Doe", "John") # Invalid, arguments in wrong order

Keyword Parameters

With keyword arguments, the name of the parameter is explicitly provided during the function call:

full_name(last="Doe", first="John") # Valid

This makes the order irrelevant since the mapping to parameters is explicit. Keyword arguments can also be combined with positional arguments:

full_name("John", last="Doe") # Valid

Generally, use positional for simplicity, keyword for clarity. Keyword arguments are required when calling functions with lots of parameters where order may be unclear.

Default Parameter Values

In Python, default values can be specified for parameters during function definition:

def logarithm(x, base=10):
  return math.log(x, base)

This allows:

Default values make parameters optional and provide sane defaults for ease of use. They also allow specifying just the non-default parameters, making the function calls concise and readable.

Variable-Length Parameters

Python provides two constructs for working with variable-length parameter lists:

  1. *args - Variable positional parameters
  2. **kwargs - Variable keyword parameters

*args - Variable Positional Parameters

The *args construct allows a function to accept any number of positional arguments:

def sum(*nums):
  total = 0
  for n in nums:
    total += n
  return total

sum(1, 5, 8) # 14
sum(1, 3) # 4

This provides flexibility since callers can pass any number of arguments without needing multiple function definitions to handle each case.

**kwargs - Variable Keyword Parameters

The **kwargs construct gathers keyword arguments into a dictionary:

def print_kwargs(**kwargs):
  for key, value in kwargs.items():
    print(f"{key}: {value}")

print_kwargs(name="John", job="Programmer")

# Output:
# name: John
# job: Programmer

This allows a function to easily accept and handle any number of keyword arguments. The key-value pairs are received in an easy to iterate dictionary kwargs.

Unpacking Parameters

To unpack elements from existing iterables into *args and **kwargs, the unpacking operator * can be used:

values = [1, 5, 8]
print_kwargs(*values) # Passes values as positional args

dict = {"name": "Mary", "age": 25}
print_kwargs(**dict) # Passes dict keys & values as kwargs

This allows passing existing data structures to functions expecting variable parameters.

Parameter Scope

Parameters and arguments exist in local function scope. Modifying a parameter only changes it within enclosing function and does not affect the original argument:

def double(num):
  num *= 2 # num changes only inside function
  return num

x = 5
print(double(x)) # 10
print(x) # 5, original x unchanged

Parameters are private to the function. However, with mutable arguments like lists or dictionaries, in-place changes to them will be visible outside since it is the same object being referenced:

def add_item(shopping_list):

mylist = ["Bread"]

print(mylist) # ["Bread", "Milk"]

So parameters act as local variables, but can cause side-effects on mutable arguments.

Common Mistakes

Some common mistakes to avoid when working with parameters:

Best Practices

To effectively leverage parameters and write clean Python code:


Properly utilizing parameters makes Python code easier to reuse, testable, and maintainable. Parameters enable decoupling code into small, single-purpose modular functions instead of monolithic ones.

Learning parameter basics like passing arguments, setting defaults, handling variable numbers of arguments, and avoiding common mistakes will level up your Python code. Applying best practices ensures your functions are flexible, robust, and Pythonic.

With this comprehensive guide, you now have a strong foundation for leveraging parameters to pass data into functions effectively in Python. The techniques covered will help you write more scalable and production-ready Python code across applications.