I'm trying to update a dictionary using a function returning a tuple where the first element is a string.
>>> def t():
... return 'ABC', 123
However the dict.update
function does not quite like it.
>>> d = {}
>>> d.update(t())
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: dictionary update sequence element #0 has length 3; 2 is required
I can also try dictionary comprehension and get the same unexpected result.
>>> d.update({k: v for k, v in t()})
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 1, in <dictcomp>
ValueError: too many values to unpack (expected 2)
The only working way is to first save returned value.
>>> x = t()
>>> d.update({x[0]: x[1]})
>>> d
{'ABC': 123}
How to explain this?
From the docs
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)
.
For example:
In [1]: d = {}
In [2]: def t(): return [('ABC', '123')]
In [3]: d.update(t())
In [4]: d
Out[4]: {'ABC': '123'}
In [5]: d2 = {}
In [6]: def t2(): return {'ABC': '123'}
In [7]: d2.update(t2())
In [8]: d2
Out[8]: {'ABC': '123'}
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