What is a pure function in Python
Pure functions in Python refer to functions that always return the same output with no side effects given the same input. Its characteristics include: 1. Determinism, that is, the same input always produces the same output; 2. No side effects, that is, no external variables, no input data, and no interaction with the outside world. For example, def add(a, b): return ab is a pure function because no matter how many times add(2, 3) is called, it always returns 5 without changing anything else in the program. In contrast, functions that modify global variables or change input parameters are non-pure functions. The advantages of pure functions are: easier to test, more suitable for concurrent execution, cache results to improve performance, and can be well matched with functional programming tools such as map() and filter(). Suggestions for writing pure functions include avoiding the use or modifying global variables, not changing input parameters, and separating I/O operations from logic. Although not all functions must be pure functions, when building complex applications, pure functions should be preferred in core logic, and side effects should be used in logging, disk saving and other scenarios.
A pure function in Python is a function that, given the same inputs, will always return the same output and has no side effects. That means it doesn't modify variables outside its scope, mutate input data, or interact with the outside world (like printing to the screen or writing to a file).
This might sound abstract at first, but once you get the idea, it makes your code more predictable and easier to test.
What Makes a Function "Pure"?
There are two main characteristics of a pure function:
- Deterministic : The same input always gives the same output.
- No Side Effects : It doesn't change anything outside itself — not global variables, not the input arguments, nothing.
Here's an example of a pure function:
def add(a, b):
return ab No matter how many times you call add(2, 3) , it'll always return 5 . And it doesn't touch anything else in your program.
Now compare that to an impure function:
count = 0
def increment():
Global count
count = 1 This function changes a variable outside of itself ( count ), so it's not pure.
Why Should You Care About Pure Functions?
Pure functions help reduce bugs and make code easier to reason about. Here are a few practical benefits:
- Easier to test : You don't have to worry about setup or cleanup because they don't rely on or change external state.
- Better for concurrency : Since they don't share state, they're safer to use in parallel or multi-threaded environments.
- Cacheable : Because the output only depends on inputs, you can cache results for performance.
They also play well with functional programming tools like map() , filter() , and libraries like functools .
How to Write More Pure Functions
It's not always possible (or necessary) to write all your code this way, but here are some habits to lean into:
- Avoid using or modifying global variables.
- Don't change input parameters — especially mutable ones like lists or dicts. Instead, create new values and return them.
- Keep I/O operations (like reading/writing files or printing) separate from logic.
For example, instead of this:
def add_item(lst, item):
lst.append(item)Do this:
def add_item(lst, item):
return lst [item]The second version doesn't change the original list — it returns a new one.
When It's Okay to Break the Rules
Let's be real — not every function needs to be pure. Sometimes you need to update state or work with external systems. That's totally fine. Just be aware of when and why you're doing it.
If you're building complex apps or working with large data pipelines, aim for purity where it matters most — core logic, transformations, and business rules. Save side effects for things like logging, saving to disk, or user interaction.
Basically that's it.
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