chapters/functional_programming.ipynb

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    "# Functional Programming\n",
    "\n",
    "Python supports multiple programming paradigms, and functional programming is one of them. So, let's break it down.\n",
    "\n",
    "What is Functional Programming?\n",
    ": Functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions. It emphasizes the application of functions to inputs to produce outputs without changing state and avoiding mutable data.\n",
    "\n",
    "## Key concepts of Functional Programming\n",
    "\n",
    "### Pure functions\n",
    "\n",
    "Pure functions are the backbone of functional programming. \n",
    "A function is considered pure if:\n",
    "- It always returns the same output for the same input.\n",
    "- It doesn't modify any external state (no side effects).\n",
    "- It doesn't depend on any external state.\n",
    "\n",
    "**Pure function example:**"
   ]
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    "def add(a, b):\n",
    "    return a + b"
   ]
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    "**Impure function example (depends on external state):**"
   ]
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    "total = 0\n",
    "def add_to_total(value):\n",
    "    global total\n",
    "    total += value\n",
    "    return total"
   ]
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    "### First class and Higher-order Functions\n",
    "\n",
    "In functional programming, functions are treated as first-class citizens. This means:\n",
    "- Functions can be assigned to variables.\n",
    "- Functions can be passed as arguments to other functions.\n",
    "- Functions can be returned from other functions.\n",
    "\n",
    "Higher-order functions are functions that can accept other functions as arguments or return functions."
   ]
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     "text": [
      "20\n"
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   "source": [
    "def apply_operation(func, x, y):\n",
    "    return func(x, y)\n",
    "\n",
    "def multiply(a, b):\n",
    "    return a * b\n",
    "\n",
    "result = apply_operation(multiply, 4, 5)\n",
    "print(result)"
   ]
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    "This concept allows for more flexible and reusable code.\n",
    "```{note}\n",
    "Not all programming languages support functional programming concepts equally. \n",
    "While Python offers excellent support for functional programming, including the ability to pass functions as arguments, this isn't universal across all languages.\n",
    "\n",
    "- Some languages (like Haskell) are built around functional programming.\n",
    "- Others (like JavaScript) have strong support for functional concepts.\n",
    "- Some languages (like older versions of Java) have limited support and may require workarounds.\n",
    "- A few languages might not support certain functional concepts at all.\n",
    "```\n",
    "\n",
    "### Recursion\n",
    "\n",
    "Recursion is a technique where a function calls itself to solve a problem. \n",
    "In functional programming, recursion is often used instead of loops to iterate over data structures.\n",
    "We have already covered recursion before, so let's not focus on it today.\n",
    "\n",
    "## Functional programming tools in Python"
   ]
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    "### `map`, `filter` and `reduce`\n",
    "\n",
    "#### `map`\n",
    "The `map()` function is a built-in Python function that applies a given function to each item in an iterable (like a `list`, `tuple`, or `set`) and returns a map object. This `map` object can be converted to a list or iterated over.\n",
    "\n",
    "Syntax:\n",
    "```python\n",
    "map(function, iterable)\n",
    "```\n",
    "\n",
    "```{note}\n",
    "Key features:\n",
    "1. It takes at least two arguments: a function and an iterable.\n",
    "2. The function is applied to every item in the iterable.\n",
    "3. It can handle multiple iterables if the function takes multiple arguments.\n",
    "4. It returns a map object, which is an iterator.\n",
    "```\n",
    "\n",
    "Examples:\n",
    "Basic usage with a single iterable:"
   ]
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     "text": [
      "[1, 4, 9, 16, 25]\n"
     ]
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   "source": [
    "def square(x):\n",
    "    return x ** 2\n",
    "\n",
    "numbers = [1, 2, 3, 4, 5]\n",
    "squared_numbers = map(square, numbers)\n",
    "print(list(squared_numbers))"
   ]
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   "source": [
    "Using map with built-in functions:"
   ]
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    "numbers = ['1', '2', '3', '4', '5']\n",
    "integers = map(int, numbers)\n",
    "print(list(integers))"
   ]
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    "Map with multiple iterables:"
   ]
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     "text": [
      "[11, 22, 33]\n"
     ]
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   "source": [
    "def add(x, y):\n",
    "    return x + y\n",
    "\n",
    "list1 = [1, 2, 3]\n",
    "list2 = [10, 20, 30]\n",
    "sums = map(add, list1, list2)\n",
    "print(list(sums))"
   ]
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    "Benefits of using `map()`:\n",
    "\n",
    "1. It provides a concise way to apply a function to all elements in an iterable.\n",
    "2. It's memory-efficient as it returns an iterator.\n",
    "3. It can make code more readable by separating the function definition from its application.\n",
    "\n",
    "When to use `map()`:\n",
    "\n",
    "1. When you need to apply the same operation to all elements in a collection.\n",
    "2. When working with large datasets, as it's more memory-efficient than list comprehensions.\n",
    "3. When you want to emphasize the functional programming style in your code."
   ]
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    "#### `filter`\n",
    "The `filter()` function is a built-in Python function that creates an iterator from elements of an iterable for which a function returns True. \n",
    "It's used to filter out elements from a sequence based on a given criteria.\n",
    "\n",
    "Syntax:\n",
    "```python\n",
    "filter(function, iterable)\n",
    "```\n",
    "\n",
    "```{note}\n",
    "Key features:\n",
    "\n",
    "1. It takes two arguments: a function and an iterable.\n",
    "2. The function should return a boolean value (`True` or `False`).\n",
    "3. It returns a filter object, which is an iterator.\n",
    "4. Elements for which the function returns True are included in the result.\n",
    "```\n",
    "Examples:\n",
    "\n",
    "Basic usage:"
   ]
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     "text": [
      "[2, 4, 6, 8, 10]\n"
     ]
    }
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   "source": [
    "def is_even(num):\n",
    "    return num % 2 == 0\n",
    "\n",
    "numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]\n",
    "even_numbers = filter(is_even, numbers)\n",
    "print(list(even_numbers))"
   ]
  },
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   "source": [
    "Using filter with built-in functions:"
   ]
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     "text": [
      "[1, True, 'hello']\n"
     ]
    }
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   "source": [
    "mixed_list = [0, 1, False, True, '', 'hello', None, [], {}]\n",
    "truthy_values = filter(bool, mixed_list)\n",
    "print(list(truthy_values))"
   ]
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    "Benefits of using `filter()`:\n",
    "\n",
    "1. It provides a concise way to select elements from an iterable based on a condition.\n",
    "2. It's memory-efficient as it returns an iterator.\n",
    "3. It separates the filtering logic from the iteration, making the code more modular.\n",
    "\n",
    "When to use `filter()`:\n",
    "\n",
    "1. When you need to select elements from a sequence based on a specific condition.\n",
    "2. When working with large datasets, as it's more memory-efficient than list comprehensions.\n",
    "3. When you want to emphasize the functional programming style in your code."
   ]
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    "#### `reduce`\n",
    "\n",
    "The `reduce()` function is part of the functools module in Python. \n",
    "It applies a function of two arguments cumulatively to the items of an iterable, from left to right, so as to reduce the iterable to a single value.\n",
    "\n",
    "Syntax:\n",
    "```python\n",
    "from functools import reduce\n",
    "reduce(function, iterable[, initializer])\n",
    "```\n",
    "\n",
    "```{note}\n",
    "Key features:\n",
    "\n",
    "1. It takes a function that accepts two arguments and an iterable.\n",
    "2. Optionally, you can provide an initializer value.\n",
    "3. It applies the function to the first two items in the iterable, then to the result and the next item, and so on.\n",
    "4. It returns a single value.\n",
    "```\n",
    "\n",
    "Examples:\n",
    "\n",
    "Basic usage - summing numbers:"
   ]
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     "text": [
      "15\n"
     ]
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   "source": [
    "from functools import reduce\n",
    "\n",
    "def add(x, y):\n",
    "    return x + y\n",
    "\n",
    "numbers = [1, 2, 3, 4, 5]\n",
    "sum_result = reduce(add, numbers)\n",
    "print(sum_result)"
   ]
  },
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   "cell_type": "markdown",
   "id": "6811cecb-9b81-419e-bbf4-5fa89ab9536a",
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   "source": [
    "Finding the maximum value:"
   ]
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     "text": [
      "9\n"
     ]
    }
   ],
   "source": [
    "def max_value(x, y):\n",
    "    return x if x > y else y\n",
    "\n",
    "numbers = [3, 7, 2, 9, 5, 1]\n",
    "maximum = reduce(max_value, numbers)\n",
    "print(maximum)"
   ]
  },
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   "cell_type": "markdown",
   "id": "1cbb3444-7d26-480e-99ee-43a2456fca01",
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   "source": [
    "Concatenating strings:"
   ]
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   "execution_count": 16,
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   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "python-is-awesome\n"
     ]
    }
   ],
   "source": [
    "def concatenate(s1, s2):\n",
    "    return s1 + '-' + s2\n",
    "\n",
    "words = ['python', 'is', 'awesome']\n",
    "result = reduce(concatenate, words)\n",
    "print(result)"
   ]
  },
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   "id": "9398cd75-043a-4adc-a63e-c57b468d0ec2",
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   "source": [
    "Using `reduce` with an initializer:"
   ]
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     "text": [
      "240\n"
     ]
    }
   ],
   "source": [
    "def multiply(x, y):\n",
    "    return x * y\n",
    "\n",
    "numbers = [1, 2, 3, 4]\n",
    "product = reduce(multiply, numbers, 10)  # 10 is the initializer\n",
    "print(product)"
   ]
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    "Benefits of using `reduce()`:\n",
    "\n",
    "1. It's useful for performing cumulative calculations on sequences.\n",
    "2. It can express certain algorithms more concisely than explicit loops.\n",
    "3. It's part of the functional programming paradigm in Python.\n",
    "\n",
    "When to use `reduce()`:\n",
    "\n",
    "1. When you need to apply a function cumulatively to a sequence.\n",
    "2. For operations like summing all numbers, finding the maximum, or concatenating all strings in a list.\n",
    "3. When you want to express a fold or accumulation operation in a functional style.\n",
    "\n",
    "````{note}\n",
    "It's worth noting that for simple operations like summing numbers or finding the maximum, Python provides built-in functions (`sum()`, `max()`) which are often more readable:\n",
    "```python\n",
    "numbers = [1, 2, 3, 4, 5]\n",
    "sum_result = sum(numbers)\n",
    "max_result = max(numbers)\n",
    "```\n",
    "````"
   ]
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    "### Lambda functions\n",
    "\n",
    "Lambda functions, also known as anonymous functions, are small, one-line functions that don't require a formal def statement. \n",
    "They're defined using the 'lambda' keyword.\n",
    "\n",
    "Here is the basic syntax of lambda function:\n",
    "```python\n",
    "lambda arguments: expression\n",
    "```\n",
    "\n",
    "Key points about lambda functions:\n",
    "- Single Expression: Lambda functions can only contain a single expression. This expression is evaluated and returned.\n",
    "- Anonymous: They don't have a name unless assigned to a variable.\n",
    "- Short-lived: Typically used for short operations that are not reused throughout the code.\n",
    "- Arguments: Can take any number of arguments but only one expression.\n",
    "\n",
    "#### Basic examples\n",
    "\n",
    "Basic lambda function:"
   ]
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    "square = lambda x: x**2\n",
    "print(square(5))"
   ]
  },
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    "Multiple arguments:"
   ]
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    {
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     "text": [
      "6\n"
     ]
    }
   ],
   "source": [
    "sum = lambda a, b, c: a + b + c\n",
    "print(sum(1, 2, 3))"
   ]
  },
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    "No arguments:"
   ]
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   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Hello, World!\n"
     ]
    }
   ],
   "source": [
    "greet = lambda: \"Hello, World!\"\n",
    "print(greet())"
   ]
  },
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   "source": [
    "With conditional expressions:"
   ]
  },
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   "execution_count": 7,
   "id": "0cf4d109-fe11-48f8-88b7-194be64ba196",
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "10\n"
     ]
    }
   ],
   "source": [
    "max = lambda a, b: a if a > b else b\n",
    "print(max(10, 5))"
   ]
  },
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   "source": [
    "```{note}\n",
    "While lambda functions are concise and useful for simple, one-off operations, it's generally better to create a regular function if you plan to name it\n",
    "```"
   ]
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    "#### Common use cases\n",
    "\n",
    "As key functions in sorting:\n",
    "Lambda functions are often used with Python's sorting methods to define custom sorting criteria. They provide a concise way to specify how elements should be compared.\n",
    "Example:"
   ]
  },
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   "execution_count": 8,
   "id": "033969e0-9e32-4db4-92f4-30735165ea13",
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[('Alice', 'A', 95), ('Charlie', 'A', 91), ('Bob', 'B', 87)]\n"
     ]
    }
   ],
   "source": [
    "students = [('Alice', 'A', 95), ('Bob', 'B', 87), ('Charlie', 'A', 91)]\n",
    "# Sort by grade, then by score\n",
    "students.sort(key=lambda student: (student[1], -student[2]))\n",
    "print(students)"
   ]
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    "Here, the lambda function creates a tuple (grade, negative score) for each student, allowing for a multi-level sort."
   ]
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    "In functional programming constructs:\n",
    "Lambda functions work well with higher-order functions like map(), filter(), and reduce(). \n",
    "They allow you to quickly define the operation to be applied to each element.\n",
    "Example:"
   ]
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    "numbers = [1, 2, 3, 4, 5]\n",
    "evens = list(filter(lambda x: x % 2 == 0, numbers))\n",
    "squares = list(map(lambda x: x**2, numbers))\n",
    "print(evens)\n",
    "print(squares)"
   ]
  },
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   "source": [
    "This approach allows for concise data transformations without defining separate functions for each operation.\n",
    "\n",
    "```{note}\n",
    "In all these cases, lambda functions shine when the operation is simple and doesn't need to be reused elsewhere in the code. \n",
    "They allow for more compact and sometimes more readable code by keeping the logic close to where it's used. \n",
    "However, for more complex operations or when the same logic is used multiple times, it's often better to define a regular named function instead.\n",
    "```"
   ]
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    "## Advanced Functional Techniques\n",
    "\n",
    "### Decorators\n",
    "\n",
    "A **decorator** is a design pattern in Python that allows you to modify the behavior of a function or a method. \n",
    "You can think of decorators as wrappers that add functionality to an existing function or method without changing its actual code.\n",
    "\n",
    "\n",
    "#### Basics about decarators\n",
    "How Decorators Work?\n",
    ": A decorator is essentially a function that takes another function as an argument, adds some code to it, and then returns a new function. When you apply a decorator to a function, you are replacing the original function with a modified version created by the decorator.\n",
    "\n",
    "Here’s the basic structure of how a decorator works:"
   ]
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   "source": [
    "def my_decorator(func):\n",
    "    def wrapper():\n",
    "        # Code to run before the original function\n",
    "        print(\"Something before the function.\")\n",
    "\n",
    "        # Call the original function\n",
    "        func()\n",
    "\n",
    "        # Code to run after the original function\n",
    "        print(\"Something after the function.\")\n",
    "\n",
    "    return wrapper"
   ]
  },
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   "cell_type": "markdown",
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   "source": [
    "In this structure:\n",
    "\n",
    "- `my_decorator` is the decorator function.\n",
    "- `func` is the original function being decorated.\n",
    "- `wrapper` is an inner function that adds behavior before and after the call to `func`.\n",
    "\n",
    "**Applying a Decorator**\n",
    "\n",
    "You apply a decorator to a function using the `@decorator_name` syntax, placed above the function definition. Here’s an example:"
   ]
  },
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   "cell_type": "code",
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   "metadata": {},
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    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Something before the function.\n",
      "Hello!\n",
      "Something after the function.\n"
     ]
    }
   ],
   "source": [
    "@my_decorator\n",
    "def say_hello():\n",
    "    print(\"Hello!\")\n",
    "\n",
    "say_hello()"
   ]
  },
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   "cell_type": "markdown",
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    "When `say_hello()` is called:\n",
    ": The `my_decorator` function is called with `say_hello` as its argument.\n",
    "`my_decorator` returns the wrapper function.\n",
    "The wrapper function is executed, which in turn calls the original `say_hello` function but also adds the extra behavior.\n",
    "\n",
    "```{admonition} Why Use Decorators?\n",
    ":class: dropdown, note\n",
    "\n",
    "Decorators are powerful because they allow you to:\n",
    "1. Add behavior: You can easily add logging, security checks, performance monitoring, or any other cross-cutting concerns to functions.\n",
    "2. Keep code DRY: You avoid repeating code by encapsulating common functionality in a decorator.\n",
    "3. Enhance readability: By separating concerns, your main function code remains clean and focused on its primary purpose.\n",
    "```\n",
    "\n",
    "#### Examples of Common Use Cases\n",
    "\n",
    "**Logging Decorator:**\n",
    "\n",
    "This decorator logs every time a function is called."
   ]
  },
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   "cell_type": "code",
   "execution_count": 20,
   "id": "3ddd649e-8997-4f0e-afd5-42dc576d6146",
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Calling add with arguments (3, 5) and {}\n"
     ]
    }
   ],
   "source": [
    "def log_decorator(func):\n",
    "    def wrapper(*args, **kwargs):\n",
    "        print(f\"Calling {func.__name__} with arguments {args} and {kwargs}\")\n",
    "        return func(*args, **kwargs)\n",
    "    return wrapper\n",
    "\n",
    "@log_decorator\n",
    "def add(x, y):\n",
    "    return x + y\n",
    "\n",
    "result = add(3, 5)"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "38318677-a5d2-4332-9c61-1362a8619298",
   "metadata": {},
   "source": [
    "**Timing Decorator:**\n",
    "\n",
    "This decorator measures how long a function takes to execute."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 22,
   "id": "b825b154-1288-4496-8724-5d17adc73916",
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Finished sleeping\n",
      "slow_function took 0.5003 seconds to execute.\n"
     ]
    }
   ],
   "source": [
    "import time\n",
    "\n",
    "def timer_decorator(func):\n",
    "    def wrapper(*args, **kwargs):\n",
    "        start_time = time.time()\n",
    "        result = func(*args, **kwargs)\n",
    "        end_time = time.time()\n",
    "        print(f\"{func.__name__} took {end_time - start_time:.4f} seconds to execute.\")\n",
    "        return result\n",
    "    return wrapper\n",
    "\n",
    "@timer_decorator\n",
    "def slow_function():\n",
    "    time.sleep(0.5)\n",
    "    print(\"Finished sleeping\")\n",
    "\n",
    "slow_function()"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "7f694f1a-e021-49a9-a33f-d25713e021b9",
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   "source": [
    "#### Summary\n",
    "\n",
    "- Decorators are functions that modify the behavior of other functions or methods.\n",
    "- They are applied using the `@decorator_name` syntax.\n",
    "- Decorators can add code before and after the original function runs, making them ideal for logging, timing, authentication, and more.\n",
    "- They help keep your code clean, reusable, and maintainable by separating concerns.\n",
    "\n",
    "\n",
    "### Partial Functions\n",
    "\n",
    "Partial functions allow you to fix a certain number of arguments of a function and generate a new function. This is useful when you need a simpler function with fewer arguments.\n",
    "\n",
    "How do they work?\n",
    ": The `functools.partial` function is used to create a new function by fixing some arguments of the original function."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 23,
   "id": "31f8291c-3862-4fc9-bc05-50bf42d92866",
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "10\n"
     ]
    }
   ],
   "source": [
    "from functools import partial\n",
    "\n",
    "def multiply(x, y):\n",
    "    return x * y\n",
    "\n",
    "# Create a new function that always multiplies by 2\n",
    "double = partial(multiply, 2)\n",
    "\n",
    "print(double(5))"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "411c958e-9846-4c28-a7c9-6961b0ab8d46",
   "metadata": {},
   "source": [
    "Here, `double` is a new function where `x` is fixed as 2, so you only need to provide the `y` argument.\n",
    "\n",
    "### Function Composition\n",
    "\n",
    "Function composition is a technique where you combine two or more functions to produce a new function. The output of one function becomes the input of another.\n",
    "\n",
    "How do they work?\n",
    ": You can compose functions manually or by using helper functions from libraries, where you pass the result of one function directly into another."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 24,
   "id": "beb584d1-6d0e-4f0b-8a72-2db1fbd0ea9e",
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "8\n"
     ]
    }
   ],
   "source": [
    "def add(x):\n",
    "    return x + 1\n",
    "\n",
    "def multiply(x):\n",
    "    return x * 2\n",
    "\n",
    "def composed_function(x):\n",
    "    return multiply(add(x))\n",
    "\n",
    "print(composed_function(3))"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "48eccb16-ed60-4a3a-8c10-275d8716a699",
   "metadata": {},
   "source": [
    "In this example, `composed_function(3)` first adds 1 to 3 (resulting in 4) and then multiplies the result by 2, giving 8.\n",
    "\n",
    "### Summary\n",
    "- Decorators are a way to modify functions without changing their code.\n",
    "- Partial Functions allow you to pre-set some arguments of a function.\n",
    "- Function Composition lets you combine functions to create new ones where the output of one is the input of another.\n",
    "\n",
    "\n",
    "## Pros and Cons of Functional Programming in Python\n",
    "\n",
    "**Pros:**\n",
    "- Immutability and Statelessness: Reduces bugs and makes code easier to reason about.\n",
    "- Modularity and Reusability: Functions can be easily composed and reused, leading to cleaner, more modular code.\n",
    "- Concise and Expressive Code: Often results in shorter, more readable code, especially for data processing.\n",
    "- Parallelism and Concurrency: Naturally suited for concurrent programming due to the lack of shared state.\n",
    "- Predictable Behavior: Pure functions lead to more predictable and reliable code.\n",
    "\n",
    "**Cons:**\n",
    "- Steeper Learning Curve: Concepts like higher-order functions and immutability can be harder to grasp.\n",
    "- Less Readable for Some: May be less intuitive for those used to imperative or object-oriented programming.\n",
    "- Performance Overhead: Can introduce inefficiencies, especially with the creation of intermediate data structures.\n",
    "- Limited Library Support: Python’s functional programming tools are not as extensive as in some other languages.\n",
    "- Debugging Challenges: Can be harder to debug due to the abstract nature of the code.\n",
    "\n",
    "**When to Use Functional Programming:**\n",
    "- Data Processing: Ideal for pipelines involving transformations and filtering.\n",
    "- Concurrency: Useful for parallel and concurrent code.\n",
    "- Small, Reusable Components: When you need modular, composable functions.\n",
    "- Predictable and Testable Code: Great for applications requiring high reliability."
   ]
  }
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