Can I modify the keys in a dictionary directly for Python?

Why does the default dictionary iterator in Python iterate through dictionary keys instead of (key, value) tuples?

• Is there any reasoning behind this except the compatibility with original dictionary API?

• Answer:

  Quoting from the http://legacy.python.org/dev/peps/pep-0234/, There has been a long discussion about whether for x in dict: should assign x the successive keys, values, or items of the       dictionary.  The symmetry between "if x in y" and "for x in y"       suggests that it should iterate over keys.  This symmetry has been       observed by many independently and has even been used to "explain"       one using the other.  This is because for sequences, "if x in y"       iterates over y comparing the iterated values to x.  If we adopt       both of the above proposals, this will also hold for       dictionaries. The argument against making "for x in dict" iterate over the keys comes mostly from a practicality point of view: scans of the       standard library show that there are about as many uses of "for x       in dict.items()" as there are of "for x in dict.keys()", with the       items() version having a small majority.  Presumably many of the       loops using keys() use the corresponding value anyway, by writing       dict[x], so (the argument goes) by making both the key and value       available, we could support the largest number of cases.  While       this is true, I (Guido) find the correspondence between "for x in       dict" and "if x in dict" too compelling to break, and there's not       much overhead in having to write dict[x] to explicitly get the       value. For fast iteration over items, use "for key, value in dict.iteritems()".  I've timed the difference between for key in dict: dict[key] and for key, value in dict.iteritems(): pass and found that the latter is only about 7% faster. Resolution: By BDFL pronouncement, for x in dict iterates over the keys, and dictionaries have iteritems(), iterkeys(), and itervalues() to return the different flavors of dictionary       iterators.