##### Anonymous function (lambda expression)

In Python, functions can be regarded as "first-class citizens". Let's review the advantages of functions:

• Reduce the amount of code duplication
• Modular code

But have we ever thought that if we need a function, which is short, and only needs to be used once (no need to call it repeatedly), do we need to define a named function?

The answer is no, here we can use anonymous functions to achieve such a function.

Let's first look at how to square a number, and how to define a function:

```def square(x):
return x**2

square(3)```

The lambda expression can be written like this:

```square = lambda x: x**2
square(3)```

According to the above example, in fact, the use of lambda expressions is still very simple, as follows:

`lambda argument1, argument2,.....: expression`

Next, the map, filter, and reduce functions introduced can be used in combination with lambda expressions to play their powerful role.

##### map function

The use of the map function is as follows:

`map(function, iterable)`

Its function is to use the function function for each element of iterable, and finally return a new traversable collection.

```a = [1,2,3,4,5]
b = map(lambda x: x*2,a)
print(list(b))
# [2, 4, 6, 8, 10]```
##### filter function

The use of the filter function is as follows:

`filter(function, iterable)`

Its function is to use the function function to judge each element of iterable, and finally return a new traversable set that is all True.

```a = [1,2,3,4,5,6]
b = filter(lambda x :x%2 == 0, a)
print(list(b))
# [2, 4, 6]```
##### reduce function

The use of the reduce function is as follows:

`reduce(function, iterable)`

The function stipulates two parameters, which means that the function operation is performed on each element of the iterable and the result of the previous operation, and finally a value is obtained. Note that we need to import reduce from functools.

```from functools import reduce

a = [1,2,3,4]
b = reduce(lambda x,y: x*y,a)
print(b)
# 24 1*2*3*4```
##### summary
• lambda expression
• map, filter and reduce functions
Reference: https://cloud.tencent.com/developer/article/1524293 Advanced Python|Talk about a few commonly used built-in functions-Cloud + Community-Tencent Cloud