Python对缩进制表符和空格的解释

Question

我决定学习一点 Python。第一个介绍说它使用缩进来分组语句。虽然最好的习惯显然是只使用其中一种，但如果我互换它们会发生什么？多少个空格将被视为等于一个制表符？或者如果混合使用制表符和空格，它会完全不起作用吗？

Answer 1

空格不被视为等同于制表符。以制表符缩进的行与以 1、2、4 个 或 8 个空格缩进的行的缩进不同。

通过反例证明(错误，或者，充其量是有限的 - 制表符!= 4 个空格):

x = 1
if x == 1:
^Iprint "fff\n"
    print "yyy\n"

'^I' 显示一个 TAB。通过 Python 2.5 运行时，出现错误:

  File "xx.py", line 4
    print "yyy\n"
                ^
IndentationError: unindent does not match any outer indentation level

由此表明在 Python 2.5 中，制表符不等于空格(尤其是不等于 4 个空格)。

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哎呀 - 尴尬；我的反例证明表明制表符不等于 4 个空格。如Alex Martelli在 comment 中指出，在 Python 2 中，制表符相当于 8 个空格，将示例改编为制表符和 8 个空格表明确实如此。

x = 1
if x != 1:
^Iprint "x is not 1\n"
        print "y is unset\n"

在 Python 2 中，此代码有效，不打印任何内容。

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在 Python 3 中，规则略有不同(如 noted 和 Antti Haapala )。比较:

• Python 2 on Indentation
• Python 3 on Indentation

Python 2 说:

First, tabs are replaced (from left to right) by one to eight spaces such that the total number of characters up to and including the replacement is a multiple of eight (this is intended to be the same rule as used by Unix). The total number of spaces preceding the first non-blank character then determines the line’s indentation. Indentation cannot be split over multiple physical lines using backslashes; the whitespace up to the first backslash determines the indentation.

Python 3 说:

Tabs are replaced (from left to right) by one to eight spaces such that the total number of characters up to and including the replacement is a multiple of eight (this is intended to be the same rule as used by Unix). The total number of spaces preceding the first non-blank character then determines the line’s indentation. Indentation cannot be split over multiple physical lines using backslashes; the whitespace up to the first backslash determines the indentation.

(除了开头的单词“First”之外，它们是相同的。)

Python 3 增加了一个额外的段落:

Indentation is rejected as inconsistent if a source file mixes tabs and spaces in a way that makes the meaning dependent on the worth of a tab in spaces; a TabError is raised in that case.

这意味着在 Python 2 中运行的 TAB 与 8-space 示例将在 Python 3 中生成 TabError。最好(在 Python 3 中是必需的)确保字符序列 block 中每一行的缩进是相同的。 PEP8说“每个缩进级别使用 4 个空格”。 (Google 的编码标准说“使用 2 个空格”。)