jf...@ms4.hinet.net wrote:
> I have a dictionary like this:
>
>>>> dct ={1: 'D', 5: 'A', 2: 'B', 3: 'B', 4: 'E'}
>
> The following code works:
> But...this one?
>
>>>> for k,v in dct: print(k,v)
> ...
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> TypeError: 'int' object is not iterable
>
> No idea what the error message means:-( Can anyone explain it? Thanks
> ahead.
for k, v in items: ...
is basically a shortcut for
for x in items:
k, v = x
The assignment with multiple names on the left side triggers what is called
"unpacking", i. e.
k, v = x
is basically a shortcut for
it = iter(x)
try:
k = next(it)
v = next(it)
except StopIteration:
raise ValueError # not enough items
try:
next(it) # too many item if this succeeds
except StopIteration:
pass
else
raise ValueError
Now remember that in
for x in dct: ...
x is actually the key and only the key -- an int in your example dict.
Then
iter(x)
raises the exception you saw:
>>> iter(42)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable
If x were (for example) a string this would succeed
it = iter("some string")
as you can iterate over the string's characters -- but you'd get a
ValueError for string lengths != 2:
>>> k, v = ""
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: need more than 0 values to unpack
>>> k, v = "ab"
>>> k
'a'
>>> v
'b'
>>> k, v = "abc"
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: too many values to unpack (expected 2)
With that information, can you predict what
for k, v in {(1, 2): "three"}: print(k, v)
will print?
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