命令与需要 () 与不需要的区别是什么? [英] What distinguishes a command from needing () vs not?

问题描述

我最近花了很长时间调试一段代码，才意识到问题是我没有在命令后包含 ().哪些命令需要 () 而哪些不需要?

I recently spent way too long debugging a piece of code, only to realize that the issue was I did not include a () after a command. What is the logic behind which commands require a () and which do not?

例如:

import pandas as pd

col1=['a','b','c','d','e']
col2=[1,2,3,4,5]

df=pd.DataFrame(list(zip(col1,col2)),columns=['col1','col2'])

df.columns

按预期返回 Index(['col1', 'col2'], dtype='object').如果我们使用 .columns() 我们会得到一个错误.

Returns Index(['col1', 'col2'], dtype='object') as expected. If we use .columns() we get an error.

其他命令则相反:

df.isna()

返回:

    col1    col2
0   False   False
1   False   False
2   False   False
3   False   False
4   False   False

但是 df.isna 返回:

<bound method DataFrame.isna of   col1  col2
0    a     1
1    b     2
2    c     3
3    d     4
4    e     5>

虽然不会抛出错误，但显然不是我们想要的.

Which, while not throwing an error, is clearly not what we're looking for.

哪些命令使用 () 哪些不使用的逻辑是什么?

我在这里以 Pandas 为例，但我认为这更普遍地与 python 相关.

I use pandas as an example here, but I think this is relevant to python more generally.

推荐答案

因为函数的参数需要括号，而变量不需要，这就是为什么它是list.append() 但它是 list.items.

Because functions need parenthesis for their arguments, while variables do not, that's why it's list.append(<item>) but it's list.items.

如果你调用一个没有括号的函数，比如 list.append 返回的是函数的描述，不是函数的描述做什么，但是描述它是什么.

If you call a function without the parenthesis like list.append what returns is a description of the function, not a description of what the function does, but a description of what it is.

对于类，调用带括号的类会启动该类的对象，而调用带括号的类则指向类>itself，这意味着如果你要执行 print(SomeClass) 你会得到 这是一个对它是什么的描述，如果你调用一个没有括号的函数，你会得到同样的响应.

As for classes, a call to a class with parenthesis initiates an object of that class, while a call to a class without the parenthesis point to the class itself, which means that if you were to execute print(SomeClass) you'd get <class '__main__.SomeClass'> which is a description of what it is, the same kind of response you'd get if you were to call a function without parenthesis.

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