Why are there no and -- operators in Python

Question

Why are there no    and -- operators in Python

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This original answer I wrote is a myth from the folklore of computing  debunked by Dennis Ritchie as  historically impossible  as noted in the letters to the editors of Communications of the ACM July 2012 doi 10 1145 2209249 2209251    The C increment decrement operators were invented at a time when the C compiler wasn t very smart and the authors wanted to be able to specify the direct intent that a machine language operator should be used which saved a handful of cycles for a compiler which might do a   load memory load 1 add store memory   instead of   inc memory    and the PDP-11 even supported  autoincrement  and  autoincrement deferred  instructions corresponding to    p and  p    respectively  See section 5 3 of the manual if horribly curious   As compilers are smart enough to handle the high-level optimization tricks built into the syntax of C  they are just a syntactic convenience now    Python doesn t have tricks to convey intentions to the assembler because it doesn t use one

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as i understood it so you won t think the value in memory is changed  in c when you do x   the value of x in memory changes  but in python all numbers are immutable hence the address that x pointed as still has x not x 1  when you write x   you would think that x change what really happens is that x refrence is changed to a location in memory where x 1 is stored or recreate this location if doe s not exists

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Of course  we could say  Guido just decided that way   but I think the question is really about the reasons for that decision  I think there are several reasons    It mixes together statements and expressions  which is not good practice  See http   norvig com python-iaq html It generally encourages people to write less readable code Extra complexity in the language implementation  which is unnecessary in Python  as already mentioned

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I think this relates to the concepts of mutability and immutability of objects  2 3 4 5 are immutable in python  Refer to the image below  2 has fixed id until this python process      x   would essentially mean an in-place increment like C  In C  x   performs in-place increments  So  x 3  and x   would increment 3 in the memory to 4  unlike python where 3 would still exist in memory    Thus in python  you don t need to recreate a value in memory  This may lead to performance optimizations    This is a hunch based answer

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I know this is an old thread  but the most common use case for   i is not covered  that being manually indexing sets when there are no provided indices  This situation is why python provides enumerate    Example   In any given language  when you use a construct like foreach to iterate over a set - for the sake of the example we ll even say it s an unordered set and you need a unique index for everything to tell them apart  say  i   0 stuff     a    b    c    d    e    f   uniquestuff      for key  val in stuff items       uniquestuff key      0  1   format val  i    i    1   In cases like this  python provides an enumerate method  e g   for i   key  val  in enumerate stuff items

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First  Python is only indirectly influenced by C  it is heavily influenced by ABC  which apparently does not have these operators  so it should not be any great surprise not to find them in Python either   Secondly  as others have said  increment and decrement are supported by    and -  already   Third  full support for a    and -- operator set usually includes supporting both the prefix and postfix versions of them   In C and C    this can lead to all kinds of  lovely  constructs that seem  to me  to be against the spirit of simplicity and straight-forwardness that Python embraces   For example  while the C statement while  t      s     may seem simple and elegant to an experienced programmer  to someone learning it  it is anything but simple   Throw in a mixture of prefix and postfix increments and decrements  and even many pros will have to stop and think a bit

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To complete already good answers on that page  Let s suppose we decide to do this  prefix    i  that would break the unary   and - operators  Today  prefixing by    or -- does nothing  because it enables unary plus operator twice  does nothing  or unary minus twice  twice  cancels itself   gt  gt  gt  i 12  gt  gt  gt    i 12  gt  gt  gt  --i 12  So that would potentially break that logic  now if one needs it for list comprehensions or lambdas  from python 3 8 it s possible with the new    assignment operator  PEP572  pre-incrementing a and assign it to b   gt  gt  gt  a   1  gt  gt  gt  b    a  a 1   gt  gt  gt  b 2  gt  gt  gt  a 2  post-incrementing just needs to make up the premature add by subtracting 1   gt  gt  gt  a   1  gt  gt  gt  b    a  a 1 -1  gt  gt  gt  b 1  gt  gt  gt  a 2

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It s not because it doesn t make sense  it makes perfect sense to define  x    as  x    1  evaluating to the previous binding of x    If you want to know the original reason  you ll have to either wade through old Python mailing lists or ask somebody who was there  eg  Guido   but it s easy enough to justify after the fact   Simple increment and decrement aren t needed as much as in other languages   You don t write things like for int i   0  i  lt  10    i  in Python very often  instead you do things like for i in range 0  10    Since it s not needed nearly as often  there s much less reason to give it its own special syntax  when you do need to increment     is usually just fine   It s not a decision of whether it makes sense  or whether it can be done--it does  and it can   It s a question of whether the benefit is worth adding to the core syntax of the language   Remember  this is four operators--postinc  postdec  preinc  predec  and each of these would need to have its own class overloads  they all need to be specified  and tested  it would add opcodes to the language  implying a larger  and therefore slower  VM engine   every class that supports a logical increment would need to implement them  on top of    and -     This is all redundant with    and -   so it would become a net loss

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The    class of operators are expressions with side effects  This is something generally not found in Python   For the same reason an assignment is not an expression in Python  thus preventing the common if  a   f            using a here      idiom   Lastly I suspect that there operator are not very consistent with Pythons reference semantics  Remember  Python does not have variables  or pointers  with the semantics known from C C

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My understanding of why python does not have    operator is following  When you write this in python a b c 1 you will get three variables  labels  pointing at same object  which value is 1   You can verify this by using id function which will return an object memory address   In  19   id a  Out 19   34019256  In  20   id b  Out 20   34019256  In  21   id c  Out 21   34019256   All three variables  labels  point to the same object  Now increment one of variable and see how it affects memory addresses   In  22  a   a   1  In  23   id a  Out 23   34019232  In  24   id b  Out 24   34019256  In  25   id c  Out 25   34019256   You can see that variable a now points to another object as variables b and c  Because you ve used a   a   1 it is explicitly clear  In other words you assign completely another object to label a  Imagine that you can write a   it would suggest that you did not assign to variable a new object but ratter increment the old one  All this stuff is IMHO for minimization of confusion  For better understanding see how python variables works   In Python  why can a function modify some arguments as perceived by the caller  but not others   Is Python call-by-value or call-by-reference  Neither   Does Python pass by value  or by reference   Is Python pass-by-reference or pass-by-value   Python  How do I pass a variable by reference   Understanding Python variables and Memory Management  Emulating pass-by-value behaviour in python  Python functions call by reference  Code Like a Pythonista  Idiomatic Python

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Clarity   Python is a lot about clarity and no programmer is likely to correctly guess the meaning of --a unless s he s learned a language having that construct   Python is also a lot about avoiding constructs that invite mistakes and the    operators are known to be rich sources of defects  These two reasons are enough not to have those operators in Python   The decision that Python uses indentation to mark blocks rather than syntactical means such as some form of begin end bracketing  or mandatory end marking is based largely on the same considerations   For illustration  have a look at the discussion around introducing a conditional operator  in C  cond   resultif   resultelse  into Python in 2005  Read at least the first message and the decision message of that discussion  which had several precursors on the same topic previously    Trivia  The PEP frequently mentioned therein is the  Python Extension Proposal  PEP 308  LC means list comprehension  GE means generator expression  and don t worry if those confuse you  they are none of the few complicated spots of Python

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Because  in Python  integers are immutable  int s    actually returns a different object    Also  with    -- you need to worry about pre- versus post- increment decrement  and it takes only one more keystroke to write x  1   In other words  it avoids potential confusion at the expense of very little gain

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It was just designed that way   Increment and decrement operators are just shortcuts for x   x   1   Python has typically adopted a design strategy which reduces the number of alternative means of performing an operation   Augmented assignment is the closest thing to increment decrement operators in Python  and they weren t even added until Python 2 0

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I always assumed it had to do with this line of the zen of python      There should be one     and preferably only one     obvious way to do it    x   and x  1 do the exact same thing  so there is no reason to have both

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Other answers have described why it s not needed for iterators  but sometimes it is useful when assigning to increase a variable in-line  you can achieve the same effect using tuples and multiple assignment   b     a becomes   a b    a 1   2   and b   a   becomes   a b   a 1  a   Python 3 8 introduces the assignment    operator  allowing us to achievefoo   a  with   foo a  a 1    foo a    is still elusive though

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I believe it stems from the Python creed that  explicit is better than implicit

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Maybe a better question would be to ask why do these operators exist in C  K amp R calls increment and decrement operators  unusual   Section 2 8page 46   The Introduction calls them  more concise and often more efficient   I suspect that the fact that these operations always come up in pointer manipulation also has played a part in their introduction  In Python it has been probably decided that it made no sense to try to optimise increments  in fact I just did a test in C  and it seems that the gcc-generated assembly uses addl instead of incl in both cases  and there is no pointer arithmetic  so it would have been just One More Way to Do It and we know Python loathes that

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This may be because  GlennMaynard is looking at the matter as in comparison with other languages  but in Python  you do things the python way  It s not a  why  question  It s there and you can do things to the same effect with x    In The Zen of Python  it is given   there should only be one way to solve a problem   Multiple choices are great in art  freedom of expression  but lousy in engineering

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I m very new to python but I suspect the reason is because of the emphasis between mutable and immutable objects within the language   Now  I know that x   can easily be interpreted as x   x   1  but it LOOKS like you re incrementing in-place an object which could be immutable   Just my guess feeling hunch

[python] Why are there no ++ and --? operators in Python?

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