1. You can update many keys on the same statement.

my_dict.update(other_dict)

In this case you don't have to know how many keys are in the other_dict. You'll just be sure that all of them will be updated on my_dict.

2. You can use any iterable of key/value pairs with dict.update

As per the documentation you can use another dictionary, kwargs, list of tuples, or even generators that yield tuples of len 2.

3. You can use the update method as an argument for functions that expect a function argument.

Example:

def update_value(key, value, update_function):
    update_function([(key, value)])

update_value("k", 3, update_on_the_db)  # suppose you have a update_on_the_db function
update_value("k", 3, my_dict.update)  # this will update on the dict

The difference is that the second method does not work:

>>> {}.update(1, 2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: update expected at most 1 arguments, got 2

dict.update() expects to find a iterable of key-value pairs, keyword arguments, or another dictionary:

Update the dictionary with the key/value pairs from other, overwriting existing keys. Return None.

update() accepts either another dictionary object or an iterable of key/value pairs (as tuples or other iterables of length two). If keyword arguments are specified, the dictionary is then updated with those key/value pairs: d.update(red=1, blue=2).

map() is a built-in method that produces a sequence by applying the elements of the second (and subsequent) arguments to the first argument, which must be a callable. Unless your key object is a callable and the value object is a sequence, your first method will fail too.

Demo of a working map() application:

>>> def key(v):
...     return (v, v)
... 
>>> value = range(3)
>>> map(key, value)
[(0, 0), (1, 1), (2, 2)]
>>> product = {}
>>> product.update(map(key, value))
>>> product
{0: 0, 1: 1, 2: 2}

Here map() just produces key-value pairs, which satisfies the dict.update() expectations.

Python has this feature built-in:

>>> d = {'b': 4}
>>> d.update({'a': 2})
>>> d
{'a': 2, 'b': 4}

Or given you're not allowed to use dict.update:

>>> d = dict(d.items() + {'a': 2}.items())   # doesn't work in python 3