How to pick just one item from a generator

Question

I have a generator function like the following   def myfunct            yield result   The usual way to call this function would be   for r in myfunct      dostuff r    My question  is there a way to get just one element from the generator whenever I like  For example  I d like to do something like   while True          if something        my element   pick just one element myfunct          dostuff my element

User · Answer

generator   myfunct   while True     my element   generator next     make sure to catch the exception thrown after the last element is taken

User · Answer

Generator is a function that produces an iterator  Therefore  once you have iterator instance  use next   to fetch the next item from the iterator  As an example  use next   function to fetch the first item  and later use for in to process remaining items     create new instance of iterator by calling a generator function items   generator function      fetch and print first item first   next items  print  first item    first     process remaining items  for item in items      print  next item    item

User · Answer

You can pick specific items using destructuring  e g    gt  gt  gt   first   middle  last    range 10   gt  gt  gt  first 0  gt  gt  gt  middle  1  2  3  4  5  6  7  8   gt  gt  gt  last 9  Note that this is going to consume your generator  so while highly readable  it is less efficient than something like next    and ruinous on infinite generators   gt  gt  gt   first   rest    itertools count

User · Answer

Create a generator using  g   myfunct     Everytime you would like an item  use  next g     or g next   in Python 2 5 or below    If the generator exits  it will raise StopIteration   You can either catch this exception if necessary  or use the default argument to next     next g  default value

User · Answer

For those of you scanning through these answers for a complete working example for Python3    well here ya go   def numgen        x   1000     while True          x    1         yield x  nums   numgen     because it must be the  same  generator  for n in range 3       numnext   next nums      print numnext    This outputs   1001 1002 1003

User · Answer

I don t believe there s a convenient way to retrieve an arbitrary value from a generator   The generator will provide a next   method to traverse itself  but the full sequence is not produced immediately to save memory   That s the functional difference between a generator and a list

User · Answer

I believe the only way is to get a list from the iterator then get the element you want from that list   l   list myfunct    l 4

User · Answer

For picking just one element of a generator use break in a for statement  or list itertools islice gen  1    According to your example  literally  you can do something like   while True          if something        for my element in myfunct              dostuff my element            break       else            do generator empty     If you want  get just one element from the  once generated  generator whenever I like    I suppose 50  thats the original intention  and the most common intention   then   gen   myfunct   while True          if something        for my element in gen            dostuff my element            break       else            do generator empty     This way explicit use of generator next   can be avoided  and end-of-input handling doesn t require  cryptic  StopIteration exception handling or extra default value comparisons   The else  of for statement section is only needed if you want do something special in case of end-of-generator   Note on next      next     In Python3 the  next   method was renamed to    next     for good reason  its considered low-level  PEP 3114   Before Python 2 6 the builtin function next   did not exist  And it was even discussed to move next   to the operator module  which would have been wise   because of its rare need and questionable inflation of builtin names   Using next   without default is still very low-level practice - throwing the cryptic StopIteration like a bolt out of the blue in normal application code openly  And using next   with default sentinel - which best should be the only option for a next   directly in builtins - is limited and often gives reason to odd non-pythonic logic readablity   Bottom line  Using next   should be very rare - like using functions of operator module  Using for x in iterator   islice  list iterator  and other functions accepting an iterator seamlessly is the natural way of using iterators on application level - and quite always possible  next   is low-level  an extra concept  unobvious - as the question of this thread shows  While e g  using break in for is conventional

[python] How to pick just one item from a generator?

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