When using if else in list comprehension, the conditional judgment needs to be placed before the expression. The basic structure is: [Expression A if condition else Expression B for element in iterable object]; for example [x if x % 2 == 0 else 0 for x in range(10)] can retain even numbers and replace odd numbers to 0; multiple conditions can be nested expressions, such as ['negative' if x
When writing Python, list comprehension is a very commonly used syntax, concise and efficient. But if you want to use if else in list comprehension, you may be a little confused at the beginning because its writing is not the same as that of ordinary if.
This article will talk about how to correctly use if else in list comprehension, so that you can write one line of code without confusing the logical order.
Basic structure: if else should be placed in front
Ordinary list comprehension with only if is very common, such as:
[x for x in range(10) if x % 2 == 0]
But if you want to add else , you can't put the conditional judgment behind it. The correct way to write it is to put if else in the front part of the expression, and the format is as follows:
[Expression A if condition else Expression B for element in iterable object]
For example, you want to keep even numbers in a list and replace odd numbers with 0:
[ x if x % 2 == 0 else 0 for x in range(10) ] # Output: [0, 0, 2, 0, 4, 0, 6, 0, 8, 0]
Pay attention to the order here, and cannot be written as:
# Error writing method [ x for x in range(10) if x % 2 == 0 else 0 ] # An error will be reported
Because the conditions can only be put in the last place when there is no else. Once else is used, the entire judgment must be put in the front.
Multiple judgments? Can be nested or multiple conditional expressions
If you want to return different values according to multiple conditions, you can nest multiple if else in list comprehension, but for readability, it is generally not recommended to nest too many layers.
For example, numbers are divided into three categories: negative numbers, zeros, and positive numbers:
[ 'negative' if x < 0 else 'zero' if x == 0 else 'positive' for x in [-2, -1, 0, 1, 2] ] # Output: ['negative', 'negative', 'zero', 'positive', 'positive']
Although this writing works, it is a bit confusing. If the judgment logic is complex, it is best to split it into functions or use traditional for loops to handle it.
Some tips in practical applications
Combined with None's judgment
For example, if you want to filter out the empty values in the list or replace them with the default values:data = [None, 1, None, 3, 4] [x if x is not None else 0 for x in data] # Output: [0, 1, 0, 3, 4]
String judgment is also possible
For example, when cleaning data, convert the empty string to None or other tags:texts = ['', 'hello', ' ', 'world'] [s.strip() if s.strip() else 'empty' for s in texts] # Output: ['empty', 'hello', 'empty', 'world']
Avoid side effects
List comprehension should be kept "pure" as much as possible and do not do side effects in it, such as modifying external variables, calling I/O, etc. Otherwise, problems will occur and difficult to debug.
Basically that's it. By mastering the position and logical order of if else in list comprehension, and in conjunction with the flexible use of some actual scenarios, you can write both concise and clear code.
The above is the detailed content of Python list comprehension with if else. For more information, please follow other related articles on the PHP Chinese website!
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