pypy - Why doesn't Python always require spaces around keywords? -


why can spaces omitted before , after key words? example, why expression 2if-1e1else 1 valid?

seems work in both cpython 2.7 , 3.3:

$ python2 python 2.7.3 (default, nov 12 2012, 09:50:25)  [gcc 4.2.1 compatible apple clang 4.1 ((tags/apple/clang-421.11.66))] on darwin type "help", "copyright", "credits" or "license" more information. >>> 2if-1e1else 1 2  $ python3 python 3.3.0 (default, nov 12 2012, 10:01:55)  [gcc 4.2.1 compatible apple clang 4.1 ((tags/apple/clang-421.11.66))] on darwin type "help", "copyright", "credits" or "license" more information. >>> 2if-1e1else 1 2

and in pypy:

$ pypy python 2.7.2 (341e1e3821ff, jun 07 2012, 15:42:54) [pypy 1.9.0 gcc 4.2.1] on darwin type "help", "copyright", "credits" or "license" more information. , different: ``pypy 1.6 released!'' >>>> 2if-1e1else 1 2

identifiers in python described as:

identifier ::= (letter|"_") (letter | digit | "_")*

hence, 2if can't identifier hence if must 2,if. similar logic applies rest of expression.

basically interpreting 2if-1e1else 1 go (the full parsing quite complicated):

2if not valid identifier, 2 matches digit digit ::= "0"..."9",if matches keyword. -1e1else, -1 unary negation (u_expr ::= power | "-" u_expr | "+" u_expr | "~" u_expr) of : ( 1 matches intpart in exponentfloat ::= (intpart | pointfloat) | exponent , e1 exponent exponent ::= ("e" | "e") ["+" | "-"] digit+.) can see expressions of form ne+|-x yields float from:

>>> type(2e3) <type 'float'>

then else seen keyword, , -1 etc..

you can peruse gammar read more it.


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