search for element in list and replace it by multiple items

Question

What is the pythonic way to search through the given list ['a', 'b', 'c'] for the element b replace it and insert multiple items b1, b2, b3 so that the list finally reads as ['a', 'b1', 'b2', 'b3', 'c']

Answer 1

Using slice notation:

>>> lst = ['a', 'b', 'c']
>>> i = lst.index('b') # This raises ValueError if there's no 'b' in the list.
>>> lst[i:i+1] = 'b1', 'b2', 'b3'
>>> lst
['a', 'b1', 'b2', 'b3', 'c']

NOTE This changes only the first matching item.

Answer 2

Alternate approach: Using itertools.chain.from_iterable

>>> b = ["b1", "b2", "b3"]
>>> a = ['a', 'b', 'c']
>>> a = [b if x=='b' else [x] for x in a] 
>>> a
['a', ['b1', 'b2', 'b3'], 'c']
>>> import itertools
>>> list(itertools.chain.from_iterable(a))
['a', 'b1', 'b2', 'b3', 'c']
>>>