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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std::insert_iterator

std::insert_iterator< _Container >(3)			     Library Functions Manual			     std::insert_iterator< _Container >(3)

NAME

       std::insert_iterator< _Container > -

SYNOPSIS

       Inherits std::iterator< output_iterator_tag, void, void, void, void >.

   Public Types
       typedef _Container container_type
       typedef void difference_type
       typedef output_iterator_tag iterator_category
       typedef void pointer
       typedef void reference
       typedef void value_type

   Public Member Functions
       insert_iterator (_Container &__x, typename _Container::iterator __i)
       insert_iterator & operator* ()
       insert_iterator & operator++ ()
       insert_iterator & operator++ (int)
       insert_iterator & operator= (const typename _Container::value_type &__value)
       insert_iterator & operator= (typename _Container::value_type &&__value)

   Protected Attributes
       _Container * container
       _Container::iterator iter

Detailed Description
   template<typename _Container>class std::insert_iterator< _Container >
       Turns assignment into insertion.

       These are output iterators, constructed from a container-of-T. Assigning a T to the iterator inserts it in the container at the iterator's
       position, rather than overwriting the value at that position.

       (Sequences will actually insert a copy of the value before the iterator's position.)

       Tip: Using the inserter function to create these iterators can save typing.

       Definition at line 587 of file stl_iterator.h.

Member Typedef Documentation
   template<typename _Container> typedef _Container std::insert_iterator< _Container >::container_type
       A nested typedef for the type of whatever container you used.

       Definition at line 596 of file stl_iterator.h.

   typedef void  std::iterator< output_iterator_tag , void , void , void , void  >::difference_type [inherited]
       Distance between iterators is represented as this type.

       Definition at line 125 of file stl_iterator_base_types.h.

   typedef output_iterator_tag	std::iterator< output_iterator_tag , void , void , void , void	>::iterator_category [inherited]
       One of the tag types.

       Definition at line 121 of file stl_iterator_base_types.h.

   typedef void  std::iterator< output_iterator_tag , void , void , void , void  >::pointer [inherited]
       This type represents a pointer-to-value_type.

       Definition at line 127 of file stl_iterator_base_types.h.

   typedef void  std::iterator< output_iterator_tag , void , void , void , void  >::reference [inherited]
       This type represents a reference-to-value_type.

       Definition at line 129 of file stl_iterator_base_types.h.

   typedef void  std::iterator< output_iterator_tag , void , void , void , void  >::value_type [inherited]
       The type 'pointed to' by the iterator.

       Definition at line 123 of file stl_iterator_base_types.h.

Constructor &; Destructor Documentation
   template<typename _Container> std::insert_iterator< _Container >::insert_iterator (_Container &__x, typename _Container::iterator__i) [inline]
       The only way to create this iterator is with a container and an initial position (a normal iterator into the container).

       Definition at line 602 of file stl_iterator.h.

Member Function Documentation
   template<typename _Container> insert_iterator& std::insert_iterator< _Container >::operator* () [inline]
       Simply returns *this.

       Definition at line 656 of file stl_iterator.h.

   template<typename _Container> insert_iterator& std::insert_iterator< _Container >::operator++ () [inline]
       Simply returns *this. (This iterator does not move.)

       Definition at line 661 of file stl_iterator.h.

   template<typename _Container> insert_iterator& std::insert_iterator< _Container >::operator++ (int) [inline]
       Simply returns *this. (This iterator does not move.)

       Definition at line 666 of file stl_iterator.h.

   template<typename _Container> insert_iterator& std::insert_iterator< _Container >::operator= (const typename _Container::value_type &__value)
       [inline]
       Parameters:
	   __value An instance of whatever type container_type::const_reference is; presumably a reference-to-const T for container<T>.

       Returns:
	   This iterator, for chained operations.

       This kind of iterator maintains its own position in the container. Assigning a value to the iterator will insert the value into the
       container at the place before the iterator.

       The position is maintained such that subsequent assignments will insert values immediately after one another. For example,

       *     // vector v contains A and Z
       *
       *     insert_iterator i (v, ++v.begin());
       *     i = 1;
       *     i = 2;
       *     i = 3;
       *
       *     // vector v contains A, 1, 2, 3, and Z
       *

       Definition at line 638 of file stl_iterator.h.

Author
       Generated automatically by Doxygen for libstdc++ from the source code.

libstdc++							  Tue Jun 10 2014			     std::insert_iterator< _Container >(3)