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一个奇怪的Python Dict表达式

https://dbader.org/blog/python-mystery-dict-expression 中看到了一个很奇怪的dict表达式:

{True: 'yes', 1: 'no', 1.0: 'maybe'}

你觉得这个表达式的最后结果会是什么?

答案是 {True: 'maybe'} ! 有没有觉得很神奇?

原因在于:

  1. 在Python中, True11.0 是相等的:

    return True == 1 == 1.0
    
    True
    

    值得一说的是,Python中的布尔类型其实就是整型的一个子集,它与整型唯一的不同就是当转换成字符串时会返回 "True""False".

  2. 不仅如此, True11.0 的hash值也是一样的:

    return hash(True) == hash(1) == hash(1.0)
    
    True
    
  3. 而对于dict来说,若两个key有相同的值和hashcode,那么这两个key认为是等价的.

    以下摘自 https://docs.python.org/3/glossary.html#term-hashable 的说明:

    An object is hashable if it has a hash value which never changes during its lifetime (it needs a __hash__() method), and can be compared to other objects (it needs an __eq__() method). Hashable objects which compare equal must have the same hash value.

    Hashability makes an object usable as a dictionary key and a set member, because these data structures use the hash value internally.

    All of Python’s immutable built-in objects are hashable, while no mutable containers (such as lists or dictionaries) are. Objects which are instances of user-defined classes are hashable by default; they all compare unequal (except with themselves), and their hash value is derived from their id().

最后有一点需要说明的是,这个问题其实是可以通过 flake8, pylint 这类工具检查到的.

比如用flake8来进行检查的话就会提示 F601 dictionary key True repeated with defferent values.

[lujun9972@F41 pythondocument]$ echo "{True: 'yes', 1: 'no', 1.0: 'maybe'}" |flake8 -
stdin:1:2: F601 dictionary key True repeated with different values
stdin:1:15: F601 dictionary key True repeated with different values
stdin:1:24: F601 dictionary key True repeated with different values