def is suitable for complex functions, supports multiple lines, document strings and nesting; lambda is suitable for simple anonymous functions and is often used in scenarios where functions are passed by parameters. The situation of selecting def: ① The function body has multiple lines; ② Document description is required; ③ Called multiple places. When choosing a lambda: ① One-time use; ② No name or document required; ③ Simple logic. Note that lambda delay binding variables may throw errors and do not support default parameters, generators, or asynchronous. In actual applications, flexibly choose according to needs and give priority to clarity.
When writing Python, functions are an infrastructure that cannot be avoided. Both def
and lambda
can define functions, but their usage scenarios, functions and the design logic behind them are actually quite different. This article does not talk about too theoretical things, but only talks about the situations you will encounter, and helps you choose the right tool.

def is a standard function definition, more comprehensive and clearer
When you need a function that is fully structured, readable, and reusable, the first choice is def
. It supports multi-line statements, document strings (docstrings), parameter type annotations, and can also nest other functions or classes.
For example:

def multiply(a, b): """Returns the product of two numbers""" return a * b
Such functions are not only easy to test and debug, but also convenient for later maintenance. If you are writing scripts, libraries, or project code, you will hardly make any mistakes with def
.
Suitable for using def:

- More than one line of function body
- Have a clear name and purpose
- Need document instructions
- Called by multiple places
lambda is a one-time expression, concise but limited
The original intention of lambda
is to quickly write a simple anonymous function, which is often used in places such as sorting and mapping where functions need to be passed as parameters.
For example, this example is very common:
numbers = [1, 2, 3, 4] squared = list(map(lambda x: x ** 2, numbers))
Here we don't need to name functions or complex logic, lambda
is very suitable.
But be aware of:
- It can contain only one expression
- No name (unless assigned to a variable)
- Docstring or complex control flows are not supported
- Overuse can reduce readability
So, don't abuse lambda to save a few lines of code . Especially in teamwork, clarity is more important than "cool".
Practical selection suggestions: look at the scene and do not force the rules
Many people are confused about which one to use, but the answer is actually very simple: determine it according to actual needs .
Let’s compare a few typical scenarios:
Custom key when sorting? Can use lambda
sorted(data, key=lambda x: x['age'])
Write a decorator? It's better to use def, because decorators usually have to handle multiple parameters, exceptions, logical branches, etc.
Need unit testing? Then it must be difficult to test with def, lambda.
Write a callback function? If it is simple, you can consider lambda, but if the callback logic becomes complicated, it is best to break it into a def function.
Some small details that are easy to ignore
Although it seems that they are writing functions, some details are easily overlooked:
lambda
can capture variables, but it is late binding, which can sometimes cause pitfalls. For example:funcs = [lambda x: x * i for i in range(3)] print([f(2) for f in funcs]) # The output is not [0, 2, 4], but [4, 4, 4]
The reason is that all lambdas refer to the same variable
i
and are not evaluated until the end.def
supports default parameter values, and can do various initialization operations inside the function, which is something that lambda cannot do.def
can be used as a generator usingyield
, or asynchronous functions can be written withasync def
, and lambda does not support these advanced features at all.
Basically that's it. def and lambda have their own uses. Don’t memorize which one is better. The key is to use it flexibly in different scenarios.
The above is the detailed content of Python def vs lambda deep dive. For more information, please follow other related articles on the PHP Chinese website!

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