#### Overview of this book

If you’re a Python developer who wants to discover how to take the power of functional programming (FP) and bring it into your own programs, then this book is essential for you, even if you know next to nothing about the paradigm. Starting with a general overview of functional concepts, you’ll explore common functional features such as first-class and higher-order functions, pure functions, and more. You’ll see how these are accomplished in Python 3.6 to give you the core foundations you’ll build upon. After that, you’ll discover common functional optimizations for Python to help your apps reach even higher speeds. You’ll learn FP concepts such as lazy evaluation using Python’s generator functions and expressions. Moving forward, you’ll learn to design and implement decorators to create composite functions. You'll also explore data preparation techniques and data exploration in depth, and see how the Python standard library fits the functional programming model. Finally, to top off your journey into the world of functional Python, you’ll at look at the PyMonad project and some larger examples to put everything into perspective.
Title Page
Packt Upsell
Contributors
Preface
Free Chapter
Understanding Functional Programming
Introducing Essential Functional Concepts
Functions, Iterators, and Generators
Working with Collections
Recursions and Reductions
The Itertools Module
More Itertools Techniques
The Functools Module
Decorator Design Techniques
Conditional Expressions and the Operator Module
A Functional Approach to Web Services
Optimizations and Improvements
Other Books You May Enjoy
Index

## Using reversed() to change the order

There are times when we need a sequence reversed. Python offers us two approaches to this: the `reversed()` function, and slices with reversed indices.

For example, consider performing a base conversion to hexadecimal or binary. The following code is a simple conversion function:

```def digits(x: int, b: int) -> Iterator[int]:
if x == 0: return
yield x % b
for d in digits(x//b, b):
yield d  ```

This function uses a recursion to yield the digits from the least significant to the most significant. The value of `x%b` will be the least significant digits of `x` in the base `b`.

We can formalize it as follows:

In many cases, we'd prefer the digits to be yielded in the reverse order. We can wrap this function with the `reversed()` function to swap the order of the digits:

```def to_base(x: int, b: int) -> Iterator[int]:
return reversed(tuple(digits(x, b)))```

### Note

The `reversed()` function produces an iterable, but the argument value must be a sequence object...