Flattening a nested sequence

• list
• flatten

>>> a = [[1, 2, 3], [4, 5, 6]]
>>> b = [i for j in a for i in j]
# [1, 2, 3, 4, 5, 6]

• How to make a flat list out of a list of lists?
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• Flatten an irregular list of lists
• Comprehension for flattening a sequence of sequences?
• Flattening a shallow list in Python