---
fid: 20240205-133748
url: https://www.w3schools.com/python/python_lists_comprehension.asp
tags: list, list comprehension, comprehension
---



# List Comprehension - A Short Guide

[Python - List Comprehension](https://www.w3schools.com/python/python_lists_comprehension.asp)


**难度**: 3

**时长**: 30 min


## Comprehension 用途

对（列表中的）每个成页应用相同的操作

## 对 每个成员应用同一种操作



### 问题： 将 names 中的每个名字改为大写

```
names = ['alice', 'tom', 'bob']
```

### solution 1: (for in)

```
upper_names = []
for name in names:
    upper_names.append(name.upper())
```

### solution 2: map

```
upper_names = list(map(str.upper, names))

upper_names = list(map(lambda x: x.upper(), names))
```



### solution 3: comprehension

```
upper_names = [name.upper() for name in names]

upper_names = (name.upper() for name in names)    # iterable
upper_names = list(upper_names)
```

### 模式总结与语法

输出是 List，且针对成员做处理，完了之后，形成一个新的 List。

```python
newlist = [expression for item  in  iterable_list
```



**练习**： 如果是想把所有名字的字母逆序，该怎么实现？试一试



## 条件过滤

### 问题： 将包含了字符 `'a'` 的 list 成员找出来，形成一个新的 List:

```python
fruits = ["apple", "banana", "cherry", "kiwi", "mango"]

newlist = []
for x in fruits:
  if "a" in x:
    newlist.append(x)
```

### if

 这符合典型的 List Comprehension 特征，但需要加一个 if 条件：

```python
fruits = ["apple", "banana", "cherry", "kiwi", "mango"]

newlist = [x for x in fruits if "a" in x] 
```

### 模式总结与语法

```python
newlist = [expression for item  in  iterable  if  condition == True]
```


The **return value** is a new list, leaving the old list unchanged.


The **condition** is like a filter that only accepts the items that valuate to `True`.

## Examples

### 不作任何处理，原样复制

```python
 newlist = [x for x in range(10)] 
```

### 对全体成员转大写

```python
 newlist = [x.upper() for x in fruits] 
```

### filtering exclude | 排除

Only accept items that are not "apple" （filtering）:

```python
newlist = [x for x in fruits if x != "apple"]
```

### 真假两种情况 (if else)

```python
newlist = [x if x != "banana" else "orange" for x in fruits] 
```

if 和 for 的位置不是固定的，当 if else 成对出现时，放在 for 前面； 否则 if 放在后面（只有过滤作用）

## 二维 comprehension

用 comprehension 计算 9x9 乘法表中的所有计算结果，共 81 个乘法

###  for in 写法

```
for i in a:
    for j in b:
        c.append(i*j)
```



### 2d comprehension


```
a = range(1, 10)
b = range(1, 10)
c = [i * j for i in a for j in b]
print(c)
len(c)
```

要点：两个 for 是顺序写法



### 带 if 条件(三角形)

```
c = [i*j for i in a for j in b if i <= j]
```