Why is a[:]=1 fundamentally different to a[:]='1'?


Please consider the two snippets of code (notice the distinction between string and integer):

a = []
a[:] = '1'


a = []
a[:] = 1

In the first case a is ['1']. In the second, I get the error TypeError: can only assign an iterable. Why would using '1' over 1 be fundamentally different here?

Asked By: Randomblue



Assigning to a slice requires an iterable on the right-hand side.

'1' is iterable, while 1 is not. Consider the following:

In [7]: a=[]

In [8]: a[:]='abc'

The result is:

In [9]: a
Out[9]: ['a', 'b', 'c']

As you can see, the list gets each character of the string as a separate item. This is a consequence of the fact that iterating over a string yields its characters.

If you want to replace a range of a‘s elements with a single scalar, simply wrap the scalar in an iterable of some sort:

In [11]: a[:]=(1,) # single-element tuple

In [12]: a
Out[12]: [1]

This also applies to strings (provided the string is to be treated as a single item and not as a sequence of characters):

In [17]: a[:]=('abc',)

In [18]: a
Out[18]: ['abc']
Answered By: NPE

'1' is a string, but it is iterable. It is like a list of characters. a[:]='1' replaces the contents of the list a with the content of the string '1'. But 1 is an integer.

Python does not change the type.


print bool(1=='1') # --> False
Answered By: guettli
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