CODEWITHSUNDEEP
python
Tairan
Tairan

Reputation: 1137

Why it works about python type conversion?

I known if I want to do type conversion in Python, I should use the built-in functions.

for example:

number = int('100')

but I am using c# style to do conversion like

number = (int)('100')

it also work, could you tell me why that is working.

and if I try to do the same thing using the code below, it does not work.

number = (int)'100'

Upvotes: 2

Views: 113

Answers (5)

Karl Knechtel
Karl Knechtel

Reputation: 61635

int('100')

This means "find the object named int, and call it as a function, passing '100' as an argument".

(int)('100')

This means the same thing, since the left-hand-side is now (int), which is the same as int, for the same reason that (3) is the same as 3.

(int)'100'

By the same principle, this is the same as int '100', which is a syntax error: you have two objects - the int class, and the string '100' - next to each other, with no operator or other indication of how they are supposed to interact.

Upvotes: 2

Stefano M
Stefano M

Reputation: 4818

In python functions are first class objects, so int and (int) are actually the same object, the built-in function int. 

>>> a = (int)
>>> a('100')
100
>>> a is int
True

You can call a function as in the example above, but not use it as a cast, c-style.

Upvotes: 0

Amit Moscovich
Amit Moscovich

Reputation: 2898

In python "int" is a class and calling "int('100')" calls the __init__ method of the int class with the argument '100'. This constructs an int object.

The expression "(int)" is no different than "int". Python has no casting/conversion operators like some other languages.

Upvotes: 0

Liquid_Fire
Liquid_Fire

Reputation: 7048

Apart from when specifying function arguments, (x) is the same as x, so (int)('100') is equivalent to int('100').

They even compile to the same bytecode:

>>> import dis
>>> dis.dis(lambda: int('100'))
  1           0 LOAD_GLOBAL              0 (int)
              3 LOAD_CONST               1 ('100')
              6 CALL_FUNCTION            1
              9 RETURN_VALUE
>>> dis.dis(lambda: (int)('100'))
  1           0 LOAD_GLOBAL              0 (int)
              3 LOAD_CONST               1 ('100')
              6 CALL_FUNCTION            1
              9 RETURN_VALUE

Upvotes: 4

Ignacio Vazquez-Abrams
Ignacio Vazquez-Abrams

Reputation: 799240

Python doesn't have "type conversion"; you are passing '100' to the int constructor. Putting parens around the type does not change its value, but removing the parens from the type's argument means that it's no longer a constructor call but rather a syntax error.

Upvotes: 6

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