Iterating over dictionaries using for loops

Question

I am a bit puzzled by the following code  d     x   1   y   2   z   3   for key in d      print  key   corresponds to   d key    What I don t understand is the key portion  How does Python recognize that it needs only to read the key from the dictionary  Is key a special word in Python  Or is it simply a variable

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This is a very common looping idiom  in is an operator  For when to use for key in dict and when it must be for key in dict keys   see David Goodger s Idiomatic Python article  archived copy

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Iterating over a dict iterates through its keys in no particular order  as you can see here   This is no longer the case in Python 3 6  but note that it s not guaranteed behaviour yet    gt  gt  gt  d     x   1   y   2   z   3   gt  gt  gt  list d    y    x    z    gt  gt  gt  d keys     y    x    z    For your example  it is a better idea to use dict items     gt  gt  gt  d items      y   2     x   1     z   3    This gives you a list of tuples  When you loop over them like this  each tuple is unpacked into k and v automatically  for k v in d items        print k   corresponds to   v   Using k and v as variable names when looping over a dict is quite common if the body of the loop is only a few lines  For more complicated loops it may be a good idea to use more descriptive names  for letter  number in d items        print letter   corresponds to   number   It s a good idea to get into the habit of using format strings  for letter  number in d items        print   0  corresponds to  1   format letter  number

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When you iterate through dictionaries using the for    in   -syntax  it always iterates over the keys  the values are accessible using dictionary key     To iterate over key-value pairs  in Python 2 use for k v in s iteritems    and in Python 3 for k v in s items

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key is simply a variable   For Python2 X   d     x   1   y   2   z   3   for my var in d      print my var   corresponds to   d my var        or better   d     x   1   y   2   z   3   for the key  the value in d iteritems        print the key   corresponds to   the value   For Python3 X   d     x   1   y   2   z   3   for the key  the value in d items        print the key   corresponds to   the value

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key is just a variable name     for key in d    will simply loop over the keys in the dictionary  rather than the keys and values   To loop over both key and value you can use the following   For Python 3 x   for key  value in d items      For Python 2 x   for key  value in d iteritems      To test for yourself  change the word key to poop   In Python 3 x  iteritems   was replaced with simply items    which returns a set-like view backed by the dict  like iteritems   but even better   This is also available in 2 7 as viewitems      The operation items   will work for both 2 and 3  but in 2 it will return a list of the dictionary s  key  value  pairs  which will not reflect changes to the dict that happen after the items   call  If you want the 2 x behavior in 3 x  you can call list d items

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Iterating over dictionaries using  for  loops  d     x   1   y   2   z   3   for key in d                How does Python recognize that it needs only to read the key from the   dictionary  Is key a special word in Python  Or is it simply a   variable    It s not just for loops  The important word here is  iterating    A dictionary is a mapping of keys to values   d     x   1   y   2   z   3     Any time we iterate over it  we iterate over the keys  The variable name key is only intended to be descriptive - and it is quite apt for the purpose   This happens in a list comprehension    gt  gt  gt   k for k in d    x    y    z     It happens when we pass the dictionary to list  or any other collection type object     gt  gt  gt  list d    x    y    z     The way Python iterates is  in a context where it needs to  it calls the   iter   method of the object  in this case the dictionary  which returns an iterator  in this case  a keyiterator object     gt  gt  gt  d   iter      lt dict keyiterator object at 0x7fb1747bee08 gt    We shouldn t use these special methods ourselves  instead  use the respective builtin function to call it  iter    gt  gt  gt  key iterator   iter d   gt  gt  gt  key iterator  lt dict keyiterator object at 0x7fb172fa9188 gt    Iterators have a   next   method - but we call it with the builtin function  next    gt  gt  gt  next key iterator   x   gt  gt  gt  next key iterator   y   gt  gt  gt  next key iterator   z   gt  gt  gt  next key iterator  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  StopIteration   When an iterator is exhausted  it raises StopIteration  This is how Python knows to exit a for loop  or a list comprehension  or a generator expression  or any other iterative context  Once an iterator raises StopIteration it will always raise it - if you want to iterate again  you need a new one    gt  gt  gt  list key iterator      gt  gt  gt  new key iterator   iter d   gt  gt  gt  list new key iterator    x    y    z     Returning to dicts  We ve seen dicts iterating in many contexts  What we ve seen is that any time we iterate over a dict  we get the keys  Back to the original example    d     x   1   y   2   z   3   for key in d     If we change the variable name  we still get the keys  Let s try it    gt  gt  gt  for each key in d          print each key     gt    d each key        x   gt  1 y   gt  2 z   gt  3   If we want to iterate over the values  we need to use the  values method of dicts  or for both together   items    gt  gt  gt  list d values     1  2  3   gt  gt  gt  list d items       x   1     y   2     z   3     In the example given  it would be more efficient to iterate over the items like this   for a key  corresponding value in d items        print a key  corresponding value    But for academic purposes  the question s example is just fine

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It s not that key is a special word  but that dictionaries implement the iterator protocol   You could do this in your class  e g  see this question for how to build class iterators   In the case of dictionaries  it s implemented at the C level   The details are available in PEP 234   In particular  the section titled  Dictionary Iterators          Dictionaries implement a tp iter slot that returns an efficient   iterator that iterates over the keys of the dictionary        This    means that we can write  for k in dict            which is equivalent to  but much faster than  for k in dict keys              as long as the restriction on modifications to the dictionary    either by the loop or by another thread  are not violated    Add methods to dictionaries that return different kinds of   iterators explicitly   for key in dict iterkeys         for value in dict itervalues         for key  value in dict iteritems              This means that for x in dict is shorthand for for x in    dict iterkeys         In Python 3  dict iterkeys    dict itervalues   and dict iteritems   are no longer supported  Use dict keys    dict values   and dict items   instead

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If you are looking for a clear and visual example  cat      name    Snowy    color    White    age   14  for key   value in cat items       print key        value   Result  name   Snowy color   White age   14

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This will print the output in sorted order by values in ascending order  d     x   3   y   1   z   2   def by value item       return item 1   for key  value in sorted d items    key by value       print key   - gt    value   Output   y - gt  1 z - gt  2 x - gt  3

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You can check the implementation of CPython s dicttype on GitHub  This is the signature of method that implements the dict iterator    PyDict Next PyObject  op  Py ssize t  ppos  PyObject   pkey               PyObject   pvalue  Py hash t  phash    CPython dictobject c

User · Answer

To iterate over keys  it is slower but better to use my dict keys    If you tried to do something like this  for key in my dict      my dict key  quot -1 quot     my dict key -1  it would create a runtime error because you are changing the keys while the program is running  If you are absolutely set on reducing time  use the for key in my dict way  but you have been warned

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I have a use case where I have to iterate through the dict to get the key  value pair  also the index indicating where I am  This is how I do it  d     x   1   y   2   z   3   for i   key  value  in enumerate d items        print i  key  value   Note that the parentheses around the key  value are important  without them  you d get an ValueError  quot not enough values to unpack quot

[python] Iterating over dictionaries using 'for' loops

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