Update Python Dictionary with Tuple of Strings to set (key, value) fails

Question

dict.update([other]) says

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).

But

>>> {}.update( ("key", "value") )
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: dictionary update sequence element #0 has length 3; 2 is required

So why does Python apparently try to use the first string of the tuple?

Answer 1

The argument needs to be an iterable of tuples (or other iterables of length two), e.g. a list

>>> d = {}
>>> d.update([("key", "value")])
>>> d
{'key': 'value'}

or a tuple, however this fails:

>>> d = {}
>>> d.update((("key", "value")))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: dictionary update sequence element #0 has length 3; 2 is required

The Python documentation on tuple again solves this mystery:

Note that it is actually the comma which makes a tuple, not the parentheses. The parentheses are optional, except in the empty tuple case, or when they are needed to avoid syntactic ambiguity.

I.e. (None) is not a tuple at all, but (None,) is:

>>> type( (None,) )
<class 'tuple'>

So this works:

>>> d = {}
>>> d.update((("key", "value"),))
>>> d
{'key': 'value'}
>>>

You can't omit the parentheses because

>>> d = {}
>>> d.update(("key", "value"),)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: dictionary update sequence element #0 has length 3; 2 is required

that would be said syntactic ambiguity (comma is function argument separator).