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']