I want to create a list of strings. The Strings should all be the same except for some elements, where they should be different. Doing this I came across a strange behaviour:
x = ['spam', 'spam', 'spam', 'spam', 'spam']
x[1] = 'buzz'
x[2] = 'buzz'
x
Out[35]: ['spam', 'buzz', 'buzz', 'spam', 'spam']
But on the other hand:
y = ['spam'] * 5
y[1:3] = ['buzz']
y
Out[36]: ['spam', 'buzz', 'spam', 'spam']
Can someone pleas tell me what I am doing wrong? Thank you very much for your help!
The assignment:
y = ['spam'] * 5
Will cause y to be a list like this:
['spam', 'spam', 'spam', 'spam', 'spam']
The array slice y[1:3] captures the elements at index 1 and 2, and looks like this:
['spam', 'spam']
Aligning the slice to the array at index 1 (where the slice starts):
['spam', 'spam', 'spam', 'spam', 'spam'] # y
['spam', 'spam'] # y[1:3]
Now, when you make the assignment y[1:3] = ['buzz'], it essentially turns this into the below:
['spam', 'buzz', 'spam', 'spam']
['buzz']
y[1:3] is a slice of the list and you replace this slice with a list composed of a single element.
spam spam spam spam spam
\ /
\ /
\ /
spam buzz spam spam
IIUC, what you want to do is replace it with a list of the same size composed of N times the new element (buzz).
You could write
y[1:3] = ['buzz', 'buzz']
# or equivalently
y[1:3] = ['buzz'] * 2
which is equivalent and more efficient than using a loop like this:
for i in [1, 2]:
y[i] = 'buzz'
# or equivalently
for i in range(1, 3):
y[i] = 'buzz'
As a sidenote, here's another pitfall. A string is an iterator, so whenever an iterator is expected and you pass a string, the string is iterated over character per character, as a list of one-character strings:
y[1:3] = 'buzz'
print(y)
# ['spam', 'b', 'u', 'z', 'z', 'spam', 'spam', 'spam']
In this case, slice [1:3] is replaced with the iterator you provided, which is 'buzz', treated as ['b', 'u', 'z', 'z'].
y[1:3] = ['buzz']
This is not the same as y[1] = 'buzz' plus y[2] = 'buzz'
Just do a for loop
for i in [1,2..whatever you need to change]:
y[i] = 'buzz'
