In Python, what does "x = 5" really do/mean? Post: 302431752

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Top Forums Programming In Python, what does "x = 5" really do/mean? Post 302431752 by Narnie on Wednesday 23rd of June 2010 01:30:20 AM

06-23-2010

Registered User

Quote:

Originally Posted by jjinno

If you read up on the Python Data Model, you will see that numbers (actually all numerics) are implemented as an immutable (aka. not-changeable) type. This would mean that you could never (using Python numerics) implement something such that 2+2=5 (for all extremely large values of 2 )

No magic really. What is happening (even with your shorthand) is a replacement operator. The right side is evaluated, and then the left side is re-pointed to the new value (so the old value is essentially lost). You have to watch out when you use this though, because the immutable/mutable types each act differently... Read this for more info...

It depends on how far you are willing to go to get a "proper solution". If you want to use the "++" or "--" in-line in Python, be prepared to write the extension in C/C++ and to recompile Python just for you. You would basically be writing "object.__iplusplus__" (or something pretty close) - Again reference the Data Model to give you an idea where to look in the Python Source.

Either way you head, I applaud you for not jumping ship at the whitespace-strictness.

Thanks. I thought it likely had to do with the fact the int, str, etc are immutable, but lists, dictionaries are mutable. This clarifies that thinking.

As far as the last comment, HAHA!!! I'm just beginning to wrap my head around Python. No way do I want to write a C/C++ extension! BECAUSE I COULDN'T, hehe. Actually being able to right x++ or ++x to increment isn't important to me.

I have since learned about __call__ and __repr__ methods. I would nave loved to be able to make an int-like variable be assigned where I could just go x=5. Then when I needed to go x.incAfter (for x++) or x.incBefore (for ++x). I think I have realized that isn't possible.

What I have (with some help from others) figured out is:

Code:

class Incrementor(object):
    def __init__(self, value=0):
        self.__value = value

    def __call__(self):
        print self.__value

    def __repr__(self):
        return str(self.__value)

    def incAfter(self):
        oldvalue = self.__value
        self.__value += 1
        return oldvalue

    def incBefore(self):
        self.__value += 1
        return self.__value

then it works the way I envisioned:

Code:

>>> x = Incrementor(5)
>>> x
5
>>> x.incAfter()
5
>>> x
6
>>> print x.incBefore()
7
>>> x
7
>>> print x.incAfter()
7
>>> x
8
>>>

Works ok for me and was a great teaching tool/exercise.

Cheerio,
Narnie

Narnie

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SHEDSKIN(1)															       SHEDSKIN(1)

NAME

       Shedskin - An experimental (restricted) Python-to-C++ Compiler

SYNOPSIS

       shedskin [OPTION]... FILE

DESCRIPTION

       Shed  Skin  is  an experimental compiler, that can translate pure, but implicitly statically typed Python (2.4-2.6) programs into optimized
       C++. It can generate stand-alone programs or extension modules that can be imported and used in larger Python programs.

       Besides the typing restriction, programs cannot freely use the Python standard library (although about 25 common modules,  such	as  random
       and re, are currently supported). Also, not all Python features, such as nested functions and variable numbers of arguments, are supported.

OPTIONS

	  -a --ann		 Output annotated source code (.ss.py)

	  -b --nobounds 	 Disable bounds checking

	  -e --extmod		 Generate extension module

	  -f --flags		 Provide alternate Makefile flags

	  -l --long		 Use long long integers

	  -m --makefile 	 Specify alternate Makefile name

	  -n --silent		 Silent mode, only show warnings

	  -o --noassert 	 Disable assert statements

	  -r --random		 Use fast random number generator (rand())

	  -s --strhash		 Use fast string hashing algorithm (murmur)

	  -w --nowrap		 Disable wrap-around checking

	  -x --traceback	 Print traceback for uncaught exceptions

	  -L --lib		 Add a library directory

0.9.2								    2012-05-05							       SHEDSKIN(1)