# dict

Python 3 dictionaries have `.keys()`, `.values()`, and `.items()` methods which return memory-efficient set-like iterator objects, not lists. (See PEP 3106.)

If your dictionaries are small, performance is not critical, and you don’t need the set-like behaviour of iterator objects from Python 3, you can of course stick with standard Python 3 code in your Py2/3 compatible codebase:

    # Assuming d is a native dict ...

    for key in d:
        # code here

    for item in d.items():
        # code here

    for value in d.values():
        # code here

In this case there will be memory overhead of list creation on Py2 for each call to `items`, `values` or `keys`.

For improved efficiency, `future.builtins` (aliased to `builtins`) provides a Python 2 `dict` subclass whose `keys()`, `values()`, and `items()` methods return iterators on all versions of Python >= 2.7. On Python 2.7, these iterators also have the same set-like view behaviour as dictionaries in Python 3. This can streamline code that iterates over large dictionaries. For example:

    from __future__ import print_function
    from builtins import dict, range

    # Memory-efficient construction:
    d = dict((i, i**2) for i in range(10**7))

    assert not isinstance(d.items(), list)

    # Because items() is memory-efficient, so is this:
    d2 = dict((v, k) for (k, v) in d.items())

As usual, on Python 3 `dict` imported from either `builtins` or `future.builtins` is just the built-in `dict` class.

## Memory-efficiency and alternatives

If you already have large native dictionaries, the downside to wrapping them in a `dict` call is that memory is copied (on both Py3 and on Py2). For example:

    # This allocates and then frees a large amount of temporary memory:
    d = dict({i: i**2 for i in range(10**7)})

If dictionary methods like `values` and `items` are called only once, this obviously negates the memory benefits offered by the overridden methods through not creating temporary lists.

The memory-efficient (and CPU-efficient) alternatives are:

- to construct a dictionary from an iterator. The above line could use a generator like this:

      d = dict((i, i**2) for i in range(10**7))

- to construct an empty dictionary with a `dict()` call using `builtins.dict` (rather than `{}`) and then update it;

- to use the `viewitems` etc. functions from `future.utils`, passing in regular dictionaries:

      from future.utils import viewkeys, viewvalues, viewitems

      for (key, value) in viewitems(hugedictionary):
          # some code here

      # Set intersection:
      d = {i**2: i for i in range(1000)}
      both = viewkeys(d) & set(range(0, 1000, 7))

      # Set union:
      both = viewvalues(d1) | viewvalues(d2)

For compatibility, the functions `iteritems` etc. are also available in `future.utils`. These are equivalent to the functions of the same names in `six`, which is equivalent to calling the `iteritems` etc. methods on Python 2, or to calling `items` etc. on Python 3.