Why isn t Python very good for functional programming

Question

I have always thought that functional programming can be done in Python  Thus  I was surprised that Python didn t get much of a mention in this question  and when it was mentioned  it normally wasn t very positive  However  not many reasons were given for this  lack of pattern matching and algebraic data types were mentioned   So my question is  why isn t Python very good for functional programming  Are there more reasons than its lack of pattern matching and algebraic data types  Or are these concepts so important to functional programming that a language that doesn t support them can only be classed as a second rate functional programming language   Keep in mind that my experience with functional programming is quite limited

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Scheme doesn t have algebraic data types or pattern matching but it s certainly a functional language  Annoying things about Python from a functional programming perspective    Crippled Lambdas  Since Lambdas can only contain an expression  and you can t do everything as easily in an expression context  this means that the functions you can define  on the fly  are limited  Ifs are statements  not expressions  This means  among other things  you can t have a lambda with an If inside it   This is fixed by ternaries in Python 2 5  but it looks ugly   Guido threatens to remove map  filter  and reduce every once in a while   On the other hand  python has lexical closures  Lambdas  and list comprehensions  which are really a  functional  concept whether or not Guido admits it   I do plenty of  functional-style  programming in Python  but I d hardly say it s ideal

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Another reason not mentioned above is that many built-in functions and methods of built-in types modify an object but do not return the modified object  If those modified objects were returned  that would make functional code cleaner and more concise  For example  if some list append some object  returned some list with some object appended

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Let me demonstrate with a piece of code taken from an answer to a  functional  Python question on SO  Python   def grandKids generation  kidsFunc  val     layer    val    for i in xrange generation       layer   itertools chain from iterable itertools imap kidsFunc  layer     return layer   Haskell   grandKids generation kidsFunc val     iterate  concatMap kidsFunc   val     generation   The main difference here is that Haskell s standard library has useful functions for functional programming  in this case iterate  concat  and

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The question you reference asks which languages promote both OO and functional programming  Python does not promote functional programming even though it works fairly well   The best argument against functional programming in Python is that imperative OO use cases are carefully considered by Guido  while functional programming use cases are not  When I write imperative Python  it s one of the prettiest languages I know  When I write functional Python  it becomes as ugly and unpleasant as your average language that doesn t have a BDFL   Which is not to say that it s bad  just that you have to work harder than you would if you switched to a language that promotes functional programming or switched to writing OO Python   Here are the functional things I miss in Python    Pattern matching Tail recursion Large library of list functions Functional dictionary class Automatic currying Concise way to compose functions Lazy lists Simple  powerful expression syntax  Python s simple block syntax prevents Guido from adding it       No pattern matching and no tail recursion mean your basic algorithms have to be written imperatively  Recursion is ugly and slow in Python  A small list library and no functional dictionaries mean that you have to write a lot of stuff yourself   No syntax for currying or composition means that point-free style is about as full of punctuation as explicitly passing arguments  Iterators instead of lazy lists means that you have to know whether you want efficiency or persistence  and to scatter calls to list around if you want persistence   Iterators are use-once  Python s simple imperative syntax  along with its simple LL1 parser  mean that a better syntax for if-expressions and lambda-expressions is basically impossible  Guido likes it this way  and I think he s right

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Guido has a good explanation of this here   Here s the most relevant part      I have never considered Python to be   heavily influenced by functional   languages  no matter what people say   or think  I was much more familiar   with imperative languages such as C   and Algol 68 and although I had made   functions first-class objects  I   didn t view Python as a functional   programming language  However  earlier   on  it was clear that users wanted to   do much more with lists and functions                It is also worth noting that even   though I didn t envision Python as a   functional language  the introduction   of closures has been useful in the   development of many other advanced   programming features  For example    certain aspects of new-style classes    decorators  and other modern features   rely upon this capability       Lastly  even though a number of   functional programming features have   been introduced over the years  Python   still lacks certain features found in      real    functional programming   languages  For instance  Python does   not perform certain kinds of   optimizations  e g   tail recursion     In general  because Python s extremely   dynamic nature  it is impossible to do   the kind of compile-time optimization   known from functional languages like   Haskell or ML  And that s fine    I pull two things out of this    The language s creator doesn t really consider Python to be a functional language   Therefore  it s possible to see  functional-esque  features  but you re unlikely to see anything that is definitively functional  Python s dynamic nature inhibits some of the optimizations you see in other functional languages   Granted  Lisp is just as dynamic  if not more dynamic  as Python  so this is only a partial explanation

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One thing that is really important for this question  and the answers  is the following  What the hell is functional programming  and what are the most important properties of it  I ll try to give my view of it   Functional programming is a lot like writing math on a whiteboard  When you write equations on a whiteboard  you do not think about an execution order  There is  typically  no mutation  You don t come back the day after and look at it  and when you make the calculations again  you get a different result  or you may  if you ve had some fresh coffee      Basically  what is on the board is there  and the answer was already there when you started writing things down  you just haven t realized what it is yet   Functional programming is a lot like that  you don t change things  you just evaluate the equation  or in this case   program   and figure out what the answer is  The program is still there  unmodified  The same with the data   I would rank the following as the most important features of functional programming  a  referential transparency - if you evaluate the same statement at some other time    and place  but with the same variable values  it will still mean the same  b  no side effect - no matter how long you stare at the whiteboard  the equation another    guy is looking at at another whiteboard won t accidentally change  c  functions are values too  which can be passed around and applied with  or to  other    variables  d  function composition  you can do h g  f and thus define a new function h     which is    equivalent to calling g f        This list is in my prioritized order  so referential transparency is the most important  followed by no side effects   Now  if you go through python and check how well the language and libraries supports  and guarantees  these aspects - then you are well on the way to answer your own question

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I would never call Python    functional    but whenever I program in Python the code invariably ends up being almost purely functional   Admittedly  that s mainly due to the extremely nice list comprehension  So I wouldn t necessarily suggest Python as a functional programming language but I would suggest functional programming for anyone using Python

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In addition to other answers  one reason Python and most other multi-paradigm languages are not well suited for true functional programming is because their compilers   virtual machines   run-times do not support functional optimization   This sort of optimization is achieved by the compiler understanding mathematical rules   For example  many programming languages support a map function or method   This is a fairly standard function that takes a function as one argument and a iterable as the second argument then applies that function to each element in the iterable     Anyways it turns out that map  foo     x     map  foo    y   is the same as map  foo    x   y     The latter case is actually faster than the former because the former performs two copies where the latter performs one   Better functional languages recognize these mathematically based relationships and automatically perform the optimization   Languages that aren t dedicated to the functional paradigm will likely not optimize

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Python is almost a functional language   It s  functional lite      It has extra features  so it isn t pure enough for some     It also lacks some features  so it isn t complete enough for some     The missing features are relatively easy to write   Check out posts like this on FP in Python

[python] Why isn't Python very good for functional programming?

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