pets = ["cat", "dog", "parrot", "hamster"]

 pets = ["cat", "monkey", "dog", "monkey", "parrot", "monkey", "hamster"]

 >>> l = ['1','2','3']
>>> '\tmonkey\t'.join(l).split('\t')
['1', 'monkey', '2', 'monkey', '3']
>>> 

  
>>> pets = ['cat', 'dog', 'parrot', 'hamster']
>>> 
>>> pets = sum(([i, 'monkey'] for i in pets), [])[:-1]
>>> pets
['cat', 'monkey', 'dog', 'monkey', 'parrot', 'monkey', 'hamster']
>>>

 from itertools import repeat
pets = ['cat', 'dog', 'parrot', 'hamster']
z = reduce(lambda x, y: x + y, zip(pets, repeat("monkey")), ())

 >>> pets = ["cat", "dog", "parrot", "hamster"]
>>> reduce(lambda x, y: x +["monkey", y] if x else [y], pets, [])
['cat', 'monkey', 'dog', 'monkey', 'parrot', 'monkey', 'hamster']

 >>> pets = ["cat", "dog", "parrot", "hamster"]
>>> for index in range(len(pets) - 1, 0, -1):
...     pets.insert(index, 'monkey')
...
>>> pets
['cat', 'monkey', 'dog', 'monkey', 'parrot', 'monkey', 'hamster']