How to len generator

Question

Python generators are very useful  They have advantages over functions that return lists  However  you could len list returning function     Is there a way to len generator function      UPDATE  Of course len list generator function     would work      I m trying to use a generator I ve created inside a new generator I m creating  As part of the calculation in the new generator it needs to know the length of the old one  However I would like to keep both of them together with the same properties as a generator  specifically - not maintain the entire list in memory as it may be very long   UPDATE 2  Assume the generator knows it s target length even from the first step  Also  there s no reason to maintain the len   syntax  Example - if functions in Python are objects  couldn t I assign the length to a variable of this object that would be accessible to the new generator

User · Answer

You can len list generator   but you could probably make something more efficient if you really intend to discard the results

User · Answer

You can use reduce   For Python 3    gt  gt  gt  import functools  gt  gt  gt  def gen            yield 1         yield 2         yield 3      gt  gt  gt  functools reduce lambda x y  x   1  gen    0    In Python 2  reduce is in the global namespace so the import is unnecessary

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Suppose we have a generator   def gen        for i in range 10           yield i   We can wrap the generator  along with the known length  in an object   import itertools class LenGen object       def   init   self gen length           self gen gen         self length length     def   call   self           return itertools islice self gen   self length      def   len   self           return self length  lgen LenGen gen 10    Instances of LenGen are generators themselves  since calling them returns an iterator   Now we can use the lgen generator in place of gen  and access len lgen  as well   def new gen        for i in lgen            yield float i  len lgen   for i in new gen        print i

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You can combine the benefits of generators with the certainty of len    by creating your own iterable object   class MyIterable object       def   init   self  n           self n   n      def   len   self           return self n      def   iter   self           self  gen   self  generator           return self      def  generator self             Put your generator code here         i   0         while i  lt  self n              yield i             i    1      def next self           return next self  gen   mi   MyIterable 100  print len mi  for i in mi      print i    This is basically a simple implementation of xrange  which returns an object you can take the len of  but doesn t create an explicit list

User · Answer

Generators have no length  they aren t collections after all   Generators are functions with a internal state  and fancy syntax   You can repeatedly call them to get a sequence of values  so you can use them in loop  But they don t contain any elements  so asking for the length of a generator is like asking for the length of a function       if functions in Python are objects  couldn t I assign the length to a   variable of this object that would be accessible to the new generator    Functions are objects  but you cannot assign new attributes to them  The reason is probably to keep such a basic object as efficient as possible   You can however simply return  generator  length  pairs from your functions or wrap the generator in a simple object like this   class GeneratorLen object       def   init   self  gen  length           self gen   gen         self length   length      def   len   self            return self length      def   iter   self           return self gen  g   some generator   h   GeneratorLen g  1  print len h   list h

User · Answer

You can use send as a hack   def counter        length   10     i   0     while i  lt  length          val    yield i          if val     length               yield length         i    1  it   counter   print it next     0 print it next     1 print it send  length     10 print it next     2 print it next     3

User · Answer

You can use len list generator function     However  this consumes the generator  but that s the only way you can find out how many elements are generated  So you may want to save the list somewhere if you also want to use the items   a   list generator function    print len a   print a 0

User · Answer

The conversion to list that s been suggested in the other answers is the best way if you still want to process the generator elements afterwards  but has one flaw  It uses O n  memory  You can count the elements in a  generator without using that much memory with   sum 1 for x in generator    Of course  be aware that this might be slower than len list generator   in common Python implementations  and if the generators are long enough for the memory complexity to matter  the operation would take quite some time  Still  I personally prefer this solution as it describes what I want to get  and it doesn t give me anything extra that s not required  such as a list of all the elements    Also listen to delnan s advice  If you re discarding the output of the generator it is very likely that there is a way to calculate the number of elements without running it  or by counting them in another manner

[python] How to len(generator())

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