Redis SELECT性能(Redis SELECT performance)

 你需要括号： 
 
(4).__str__()
 
 问题是词法学家认为“4.” 将是一个浮点数。 
 
 此外，这样做： 
 
x = 4
x.__str__()

So you think you can 
 
    dance  
  floating-point?
 
123 is just as much of an object as 3.14, the "problem" lies within the grammar rules of the language; the parser thinks we are about to define a float — not an int with a trailing method call.
 
We will get the expected behavior if we wrap the number in parenthesis, as in the below.
 
>>> (123).__str__()
'123'
 
Or if we simply add some whitespace after 123:
 
>>> 123 .__str__()
'123'
 

 
The reason it does not work for 123.__str__() is that the dot following the 123 is interpreted as the decimal-point of some partially declared floating-point.
 
>>> 123.__str__()
  File "", line 1
    123.__str__()
              ^
SyntaxError: invalid syntax
 
The parser tries to interpret __str__() as a sequence of digits, but obviously fails — and we get a SyntaxError basically saying that the parser stumbled upon something that it did not expect.
 

 
 
Elaboration
 
When looking at 123.__str__() the python parser could use either 3 characters and interpret these 3 characters as an integer, or it could use 4 characters and interpret these as the start of a floating-point.
 
123.__str__()
^^^ - int
 
123.__str__()
^^^^- start of floating-point
 
Just as a little child would like as much cake as possible on their plate, the parser is greedy and would like to swallow as much as it can all at once — even if this isn't always the best of ideas —as such the latter ("better") alternative is chosen.
 
When it later realizes that __str__() can in no way be interpreted as the decimals of a floating-point it is already too late; SyntaxError.
 
 
 
Note
 
 
 123 .__str__() # works fine
 
 
In the above snippet, 123  (note the space) must be interpreted as an integer since no number can contain spaces. This means that it is semantically equivalent to (123).__str__().
 
 
 
 
Note
 
 
 123..__str__() # works fine
 
 
The above also works because a number can contain at most one decimal-point, meaning that it is equivalent to (123.).__str__().
 
 

 
 
For the language-lawyers
 
This section contains the lexical definition of the relevant literals.
 
Lexical analysis - 2.4.5 Floating point literals 
 
floatnumber   ::=  pointfloat | exponentfloat
pointfloat    ::=  [intpart] fraction | intpart "."
exponentfloat ::=  (intpart | pointfloat) exponent
intpart       ::=  digit+
fraction      ::=  "." digit+
exponent      ::=  ("e" | "E") ["+" | "-"] digit+
 
Lexical analysis - 2.4.4 Integer literals 
 
integer        ::=  decimalinteger | octinteger | hexinteger | bininteger
decimalinteger ::=  nonzerodigit digit* | "0"+
nonzerodigit   ::=  "1"..."9"
digit          ::=  "0"..."9"
octinteger     ::=  "0" ("o" | "O") octdigit+
hexinteger     ::=  "0" ("x" | "X") hexdigit+
bininteger     ::=  "0" ("b" | "B") bindigit+
octdigit       ::=  "0"..."7"
hexdigit       ::=  digit | "a"..."f" | "A"..."F"
bindigit       ::=  "0" | "1"