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

Question

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

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

and

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?

Answer 1

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'] 

Answer 2

'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.

Example:

print bool(1=='1') # --> False