python删除列表理解中的字典键 [英] python delete dict keys in list comprehension

问题描述

为什么以下旨在删除字典中多个键的表达式无效？ （事件是字典）

Why is the following expression, aiming at deleting multiple keys in a dict, invalid? (event is a dict)

[del event[key] for key in ['selected','actual','previous','forecast']]

什么

推荐答案

您完全不应该使用列表理解 强>在这里。列表推导非常适合构建值列表，因此不应将其用于常规循环。使用列表推导来解决副作用是浪费一个完美的列表对象的内存。

You should not use a list comprehension at all here. List comprehensions are great at building a list of values, and should not be used for general looping. Using a list comprehension for the side-effects is a waste of memory on a perfectly good list object.

列表推导也是表达式，因此只能包含其他表达式。 del 是一条语句，不能在表达式内使用。

List comprehensions are also expressions, so can only contain other expressions. del is a statement and can't be used inside an expression.

只需使用 for 循环：

# use a tuple if you need a literal sequence; stored as a constant
# with the code object for fast loading
for key in ('selected', 'actual', 'previous', 'forecast'):
    del event[key]

或使用字典理解力重建字典：

or rebuild the dictionary with a dictionary comprehension:

# Use a set for fast membership testing, also stored as a constant
event = {k: v for k, v in event.items()
         if k not in {'selected', 'actual', 'previous', 'forecast'}}

后者会创建一个全新的字典，因此对同一对象的其他现有引用将看不到任何更改。

The latter creates an entirely new dictionary, so other existing references to the same object won't see any changes.

如果必须在表达式中使用键删除，则可以可以使用 object .__ delitem __（key），但这不是地方；您最终会得到一个列表，其中包含 None 个对象，并且您会立即丢弃该列表。

If you must use key deletion in an expression, you can use object.__delitem__(key), but this is not the place; you'd end up with a list with None objects as a result, a list you discard immediately.

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