✏ EDIT: _itertools.py

from itertools import filterfalse

def unique_everseen(iterable, key=None):
    "List unique elements, preserving order. Remember all elements ever seen."
    # unique_everseen('AAAABBBCCDAABBB') --> A B C D
    # unique_everseen('ABBCcAD', str.lower) --> A B C D
    seen = set()
    seen_add = seen.add
    if key is None:
        for element in filterfalse(seen.__contains__, iterable):
            seen_add(element)
            yield element
    else:
        for element in iterable:
            k = key(element)
            if k not in seen:
                seen_add(k)
                yield element

# copied from more_itertools 8.8
def always_iterable(obj, base_type=(str, bytes)):
    """If *obj* is iterable, return an iterator over its items::

>>> obj = (1, 2, 3)
        >>> list(always_iterable(obj))
        [1, 2, 3]

If *obj* is not iterable, return a one-item iterable containing *obj*::

>>> obj = 1
        >>> list(always_iterable(obj))
        [1]

If *obj* is ``None``, return an empty iterable:

>>> obj = None
        >>> list(always_iterable(None))
        []

By default, binary and text strings are not considered iterable::

>>> obj = 'foo'
        >>> list(always_iterable(obj))
        ['foo']

If *base_type* is set, objects for which ``isinstance(obj, base_type)``
    returns ``True`` won't be considered iterable.

>>> obj = {'a': 1}
        >>> list(always_iterable(obj))  # Iterate over the dict's keys
        ['a']
        >>> list(always_iterable(obj, base_type=dict))  # Treat dicts as a unit
        [{'a': 1}]

Set *base_type* to ``None`` to avoid any special handling and treat objects
    Python considers iterable as iterable:

>>> obj = 'foo'
        >>> list(always_iterable(obj, base_type=None))
        ['f', 'o', 'o']
    """
    if obj is None:
        return iter(())

if (base_type is not None) and isinstance(obj, base_type):
        return iter((obj,))

try:
        return iter(obj)
    except TypeError:
        return iter((obj,))

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