class是python语言关键字吗,为什么不能属性名称是Python的关键字？

There is a restriction on the syntax of attribute access, in Python (at least in the CPython 2.7.2 implementation):

>>> class C(object): pass

>>> o = C()

>>> o.x = 123 # Works

>>> o.if = 123

o.if = 123

^

SyntaxError: invalid syntax

My question is twofold:

Is there a fundamental reason why using Python keyword attribute names (as in o.if = 123) is forbidden?

Is/where is the above restriction on attribute names documented?

It would make sense to do o.class = …, in one of my programs, and I am a little disappointed not to be able to do it (o.class_ would work, but it does not look as simple).

PS: The problem is obviously that if and class are Python keywords. The question is why using keywords as attribute names would be forbidden (I don&＃8217;t see any ambiguity in the expression o.class = 123), and whether this is documented.

解决方案

Because parser is simpler when keywords are always keywords, and not contextual (e.g. if is a keyword when on the statement level, but just an identifier when inside an expression — for if it&＃8217;d be double hard because of X if C else Y, and for is used in list comprehensions and generator expressions).

So the code doesn&＃8217;t even get to the point where there&＃8217;s attribute access, it&＃8217;s simply rejected by the parser, just like incorrect indentation (which is why it&＃8217;s a SyntaxError, and not AttributeError or something). It doesn&＃8217;t differentiate whether you use if as an attribute name, a variable name, a function name, or a type name. It can never be an identifier, simply because parser always assigns it &＃8220;keyword&＃8221; label and makes it a different token than identifiers.

It&＃8217;s the same in most languages, and language grammar (+ lexer specification) is the documentation for that. Language spec mentions it explicitly. It also doesn&＃8217;t change in Python 3.

Also, just because you can use setattr or __dict__ to make an attribute with a reserved name, doesn&＃8217;t mean you should. Don&＃8217;t force yourself/API user to use getattr instead of natural attribute access. getattr should be reserved for when access to a variable attribute name is needed.