Update method in Python dictionary

Question

I was trying to update values in my dictionary, I came across 2 ways to do so:

product.update(map(key, value))  product.update(key, value) 

What is the difference between them?

Answer 1

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.