Merge pull request #393 from realpython/python-flatten-list

Sample code for the article on flattening list

Author: KateFinegan

python-flatten-list/README.md

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+# How to Flatten a List of Lists in Python
+
+This folder provides the code examples for the Real Python tutorial [How to Flatten a List of Lists in Python](https://realpython.com/python-flatten-list/).

python-flatten-list/flatten.py

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+from functools import reduce
+from itertools import chain
+from operator import add, concat, iconcat
+
+
+def flatten_extend(matrix):
+    flat_list = []
+    for row in matrix:
+        flat_list.extend(row)
+    return flat_list
+
+
+def flatten_concatenation(matrix):
+    flat_list = []
+    for row in matrix:
+        flat_list += row
+    return flat_list
+
+
+def flatten_comprehension(matrix):
+    return [item for row in matrix for item in row]
+
+
+def flatten_chain(matrix):
+    return list(chain.from_iterable(matrix))
+
+
+def flatten_reduce_lambda(matrix):
+    return list(reduce(lambda x, y: x + y, matrix, []))
+
+
+def flatten_reduce_add(matrix):
+    return reduce(add, matrix, [])
+
+
+def flatten_reduce_concat(matrix):
+    return reduce(concat, matrix, [])
+
+
+def flatten_reduce_iconcat(matrix):
+    return reduce(iconcat, matrix, [])
+
+
+def flatten_sum(matrix):
+    return sum(matrix, [])

python-flatten-list/flatten_numpy_flatten.py

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+import numpy as np
+
+matrix = np.array(
+    [
+        [9, 3, 8, 3],
+        [4, 5, 2, 8],
+        [6, 4, 3, 1],
+        [1, 0, 4, 5],
+    ]
+)
+
+matrix.flatten()

python-flatten-list/performance.py

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+from timeit import timeit
+
+import flatten  # noqa
+
+SIZE = 1000
+TO_MS = 1000
+NUM = 10
+FUNCTIONS = [
+    "flatten_extend",
+    "flatten_concatenation",
+    "flatten_comprehension",
+    "flatten_chain",
+    "flatten_reduce_lambda",
+    "flatten_reduce_add",
+    "flatten_reduce_concat",
+    "flatten_reduce_iconcat",
+    "flatten_sum",
+]
+
+matrix = [list(range(SIZE))] * SIZE
+
+results = {
+    func: timeit(f"flatten.{func}(matrix)", globals=globals(), number=NUM)
+    for func in FUNCTIONS
+}
+
+print(f"Time to flatten a {SIZE}x{SIZE} matrix (in milliseconds):\n")
+
+for func, time in sorted(results.items(), key=lambda result: result[1]):
+    print(f"{func + '()':.<30}{time * TO_MS / NUM:.>7.2f} ms")