Add materials for functional programming update

functional-programming-python/README.md

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+# Functional Programming in Python: When and How to Use It
+
+This folder contains the sample code used in the RealPython tutorial [Functional Programming in Python: When and How to Use It](https://realpython.com/python-functional-programming/).
+
+## Run the Scrips
+
+You can run the individual files as Python scripts:
+
+```sh
+$ python file_name.py
+```
+
+You'll see some output printed to your terminal.
+
+For more context, please read the associated tutorial.
+
+## About the Author
+
+Martin Breuss - Email: martin@realpython.com
+
+## License
+
+Distributed under the MIT license. See ``LICENSE`` for more information.

functional-programming-python/filter_iterables.py

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+"""
+Example use cases of Python's `filter()` function.
+"""
+
+
+# Keep only high numbers.
+def greater_than_100(x):
+    return x > 100
+
+
+print(list(filter(greater_than_100, [1, 111, 2, 222, 3, 333])))
+print(list(filter(lambda x: x > 100, [1, 111, 2, 222, 3, 333])))
+
+
+# Filter out odd numbers.
+def is_even(x):
+    return x % 2 == 0
+
+
+print(list(filter(is_even, range(10))))
+print(list(filter(lambda x: x % 2 == 0, range(10))))
+
+# Keep only uppercase strings.
+animals = ["cat", "Cat", "CAT", "dog", "Dog", "DOG", "emu", "Emu", "EMU"]
+
+
+def all_caps(s):
+    return s.isupper()
+
+
+print(list(filter(all_caps, animals)))
+print(list(filter(lambda s: s.isupper(), animals)))
+
+# Remove empty strings.
+animals_and_empty_strings = ["", "cat", "dog", "", ""]
+
+print(list(filter(lambda s: s, animals_and_empty_strings)))

functional-programming-python/function_scopes.py

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+"""
+Example of a nested function scope.
+"""
+
+
+def outer():
+    def inner():
+        print("I am function inner()!")
+
+    # Function outer() returns function inner().
+    return inner
+
+
+function = outer()
+
+print(function)
+print(function())
+print(outer()())

functional-programming-python/functions_as_arguments.py

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+"""
+Examples of using a function object as an argument.
+"""
+
+animals = ["ferret", "vole", "dog", "gecko"]
+print(sorted(animals))
+
+animals = ["ferret", "vole", "dog", "gecko"]
+print(sorted(animals, key=len))
+
+animals = ["ferret", "vole", "dog", "gecko"]
+print(sorted(animals, key=len, reverse=True))
+
+
+def reverse_len(s):
+    return -len(s)
+
+
+print(sorted(animals, key=reverse_len))
+
+# Using a lambda expression
+sorted(animals, key=lambda s: -len(s))

functional-programming-python/functions_overview.py

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+"""
+Examples of using functions as objects.
+"""
+
+
+# Call a function.
+def func():
+    print("I am function func()!")
+
+
+print(func())
+
+
+# Assign it to a new name.
+another_name = func
+another_name()
+
+# Show a string representation of a function object.
+print("cat", func, 42)
+
+# Access a function object in a list.
+objects = ["cat", func, 42]
+objects[1]
+
+print(objects[1]())
+
+# Use a function object as a key in a dictionary.
+# Note that this is probably not a good idea.
+d = {"cat": 1, func: 2, 42: 3}
+print(d[func])
+
+
+# Pass a function as an argument.
+def inner():
+    print("I am function inner()!")
+
+
+def outer(function):
+    function()
+
+
+print(outer(inner))

functional-programming-python/higher_order_reduce.py

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+"""
+Example implementations of using `reduce()` to create
+functional versions of `map()` and `filter()`.
+"""
+
+# Compare `map()` to `custom_map()`.
+numbers = [1, 2, 3, 4, 5]
+
+print(list(map(str, numbers)))
+
+
+def custom_map(function, iterable):
+    from functools import reduce
+
+    return reduce(
+        lambda items, value: items + [function(value)],
+        iterable,
+        [],
+    )
+
+
+print(list(custom_map(str, numbers)))
+
+
+# Compare `filter()` to `custom_filter()`.
+numbers = list(range(10))
+
+
+def is_even(x):
+    return x % 2 == 0
+
+
+print(list(filter(is_even, numbers)))
+
+
+def custom_filter(function, iterable):
+    from functools import reduce
+
+    return reduce(
+        lambda items, value: items + [value] if function(value) else items,
+        iterable,
+        [],
+    )
+
+
+print(list(custom_filter(is_even, numbers)))

functional-programming-python/lambda_expressions.py

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+"""
+Examples of writing lambda expressions.
+"""
+
+
+# Reverse a string.
+def reverse(s):
+    return s[::-1]
+
+
+reverse("I am a string")
+
+
+reverse = lambda s: s[::-1]  # noqa E731
+print(reverse("I am a string"))
+
+print((lambda s: s[::-1])("I am a string"))
+
+
+# Calculate the average of three numbers.
+print((lambda x1, x2, x3: (x1 + x2 + x3) / 3)(9, 6, 6))
+print((lambda x1, x2, x3: (x1 + x2 + x3) / 3)(1.4, 1.1, 0.5))
+
+# Return a fixed value.
+forty_two_producer = lambda: 42  # noqa E731
+print(forty_two_producer())
+
+# Use a conditional expression.
+print((lambda x: "even" if x % 2 == 0 else "odd")(2))
+print((lambda x: "even" if x % 2 == 0 else "odd")(3))

functional-programming-python/map_multiple.py

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+"""
+Example of using multiple iterables as arguments to `map()`.
+"""
+
+
+def add_three(a, b, c):
+    return a + b + c
+
+
+print(list(map(add_three, [1, 2, 3], [10, 20, 30], [100, 200, 300])))
+
+# Using a lambda expression.
+print(
+    list(
+        map(
+            (lambda a, b, c: a + b + c),
+            [1, 2, 3],
+            [10, 20, 30],
+            [100, 200, 300],
+        )
+    )
+)

functional-programming-python/map_single.py

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+"""
+Examples of using `map()` with a single iterable.
+"""
+
+
+def reverse(s):
+    return s[::-1]
+
+
+print(reverse("I am a string"))
+
+animals = ["cat", "dog", "hedgehog", "gecko"]
+
+# Use `map()` to create an iterator.
+iterator = map(reverse, animals)
+print(iterator)
+
+# Consume the iterator with a for loop.
+iterator = map(reverse, animals)
+for i in iterator:
+    print(i)
+
+# Consume the iterator with a call to `list()`.
+iterator = map(reverse, animals)
+print(list(iterator))
+
+
+# Use a lambda expression.
+iterator = map(lambda s: s[::-1], animals)
+print(list(iterator))
+
+
+# All the code in a single line.
+print(list(map(lambda s: s[::-1], ["cat", "dog", "hedgehog", "gecko"])))
+
+# Convert integers to strings before concatenating them.
+print("+".join(map(str, [1, 2, 3, 4, 5])))

functional-programming-python/reduce_with_initializer.py

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+"""
+Examples of using `reduce()` with an initializer value.
+"""
+
+from functools import reduce
+
+
+def add_two_values(x, y):
+    return x + y
+
+
+# Initialize with the value `100`.
+# (100 + 1 + 2 + 3 + 4 + 5)
+print(reduce(add_two_values, [1, 2, 3, 4, 5], 100))
+
+# Use a lambda expression.
+print(reduce(lambda x, y: x + y, [1, 2, 3, 4, 5], 100))
+
+# Use `sum()` and add a value instead.
+print(100 + sum([1, 2, 3, 4, 5]))

functional-programming-python/reduce_with_two.py

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+"""
+Examples of using `reduce()` with two arguments.
+"""
+
+from functools import reduce
+
+
+# Sum elements in an iterable.
+def add_two_values(x, y):
+    return x + y
+
+
+print(reduce(add_two_values, [1, 2, 3, 4, 5]))
+print(sum([1, 2, 3, 4, 5]))
+
+
+# Concatenate strings.
+print(reduce(add_two_values, ["cat", "dog", "hedgehog", "gecko"]))
+print("".join(["cat", "dog", "hedgehog", "gecko"]))
+
+
+# Calculate the factorial with `reduce()`.
+def multiply(x, y):
+    return x * y
+
+
+def factorial_with_reduce(n):
+    from functools import reduce
+
+    return reduce(multiply, range(1, n + 1))
+
+
+print(factorial_with_reduce(4))  # 1 * 2 * 3 * 4
+print(factorial_with_reduce(6))  # 1 * 2 * 3 * 4 * 5 * 6
+
+
+# Calculate the factorial with `math.factorial()`.
+from math import factorial  # noqa E402
+
+print(factorial(4))
+print(factorial(6))
+
+
+# Identify the maximum value.
+print(max([23, 49, 6, 32]))
+
+
+def greater(x, y):
+    return x if x > y else y
+
+
+print(reduce(greater, [23, 49, 6, 32]))
+
+
+# Use lambda expressions to complete the tasks.
+print(reduce(lambda x, y: x + y, [1, 2, 3, 4, 5]))
+print(reduce(lambda x, y: x + y, ["foo", "bar", "baz", "quz"]))
+print(reduce((lambda x, y: x if x > y else y), [23, 49, 6, 32]))