对 Python 语法不够了解导致的 bug

对 Python 语法不够了解导致的 bug。

`in`

'20' in '11264,6144,4096,3072,2048,1024,300,30'
Out[7]: 
True

a_list = '11264,6144,4096,3072,2048,1024,300,30'.split(',')
a_list
Out[10]: 

['11264', '6144', '4096', '3072', '2048', '1024', '300', '30']
'20' in a_list
Out[11]: 
False

然后我去搜索 python `in`，有很多噪音，因为 `in` 这个词出现太多了。然后想到 Python 自带的 help，于是我在Python shell 中写 

help('in')

Membership test operations
**************************
The operators "in" and "not in" test for membership. "x in s"
evaluates to true if *x* is a member of *s*, and false otherwise. "x
not in s" returns the negation of "x in s". All built-in sequences
and set types support this as well as dictionary, for which "in" tests
whether the dictionary has a given key. For container types such as
list, tuple, set, frozenset, dict, or collections.deque, the
expression "x in y" is equivalent to "any(x is e or x == e for e in
y)".
For the string and bytes types, "x in y" is true if and only if *x* is
a substring of *y*. An equivalent test is "y.find(x) != -1". Empty
strings are always considered to be a substring of any other string,
so """ in "abc"" will return "True".
For user-defined classes which define the "__contains__()" method, "x
in y" is true if and only if "y.__contains__(x)" is true.
For user-defined classes which do not define "__contains__()" but do
define "__iter__()", "x in y" is true if some value "z" with "x == z"
is produced while iterating over "y". If an exception is raised
during the iteration, it is as if "in" raised that exception.
Lastly, the old-style iteration protocol is tried: if a class defines
"__getitem__()", "x in y" is true if and only if there is a non-
negative integer index *i* such that "x == y[i]", and all lower
integer indices do not raise "IndexError" exception. (If any other
exception is raised, it is as if "in" raised that exception).
The operator "not in" is defined to have the inverse true value of
"in".
Related help topics: SEQUENCEMETHODS

出现的解释很有帮助。所以对于 built-in 的关键字或者函数，我们应首先使用自带的 help 进行查询，而不是去谷歌搜索。

引申出  is 与 == 的区别　　

5 is 5
Out[16]: 
True
5 == 5
Out[17]: 
True
nan = float('NaN')
nan is nan
Out[19]: 
True
nan == nan
Out[20]: 
False

　　

参考

python-in-operator-use-cases