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Page history last edited by PBworks 15 years, 3 months ago

Assigment in C sense does not exist: name binding



Whether Python has "variables" depends on your perspective. Python

certainly does *not* have variables with anything like the semantics of

C/C++ variables. For that reason, it's often convenient to shift the

vocabulary to avoid misunderstading. However, the vast majority of

Python programmers do use "variable" in casual conversation (I certainly

do); it's only when trying to discuss the Python object model that

there's a strong tendency to switch to using "names".



"variable" names are references to objects


Since all "variable" names in Python are references to objects, anything accessed using a name is accessed by reference?


Anybody using the terms variable, reference or call-by-value is most likely explaining Python the wrong way


http://effbot.org/zone/python-objects.htm (name - binding to value of some type)

http://effbot.org/zone/call-by-object.htm - "call by sharing"


But you must be careful about what is meant by "changes to parameters". Assigning a new value to a parameter name (inside the function, a parameter is just a local variable) does not change the original object--it only rebinds the local variable to a new object.


In the following function, a is rebound with an assignment statement, while b is mutated, i.e., changed, with an assignment statement.


def f(a, b):

a = 12

b.value = 14


Argument a will never be changed, while argument b will be. Python's argument passing semantics are extremely simple. It's the assignment statement that's tricky: some assignments mutate/change objects, and some only rebind names.


multiple initialization and set-and-test


x = y = x = 3 # works: <==> (x, y, z) = (3, 3, 3)

and if x=read(): # set-and-test, does not work


(in C, x = y = 3: '=' is operator which returns value, assignment is side effect)


but name binding in Python (and binding to container

items too) doesn't "return a value".


Since Python objects, names, their operations, and the syntax used

for it resemble very closely (but not completely) what is called

"variable" in other languages, Python names are mostly called

"variables", too.


But there is this one thing of changing mutable objects which may

also change the "value" of other "variables". This can only be

understood if your nomenclature is strictly correct.


Python has "variables". It's just sometimes a good advice for people

coming from languages like C to forget about that term for a while because

they have the wrong impression of what "variable" means. A "variable" in

programming languages is composed of a name, a memory location, possibly a

type and a value. In C-like languages, where you put values in named and

typed "boxes", the memory location and type are attached to the name. In

Python both belong to the value.


Your problem is with the semantic of "modifying". In Python, (re)binding

a name and mutating an object are two very distinct things.


You just can't substitute the term "name" with "variable" and

expect it to behave like in C. Python names are very similar to automatically

dereferenced pointer variables in C++.

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