Writing cryptography-related software in Python requires using a cryptography module. These modules contain implementations of the most popular cryptography algorithms such as encryption / decryption with AES, hashing with SHA, pseudorandom number generators, and much, much more, either in pure python, or as a wrapper around popular cryptographic libraries. Furthermore, since Python supports bignums out of the box, it is an excellent platform for experimenting with cryptographic algorithms.

However, before embarking on this journey, total beginners are strongly encouraged to take a serious "Cryptography 101" course. This is essential to get an overview of the field, and to avoid all too popular beginners mistakes. After all, unlike programming, faulty cryptographic applications don't generate compiler error messages, yet they are still broken. To illustrate: the example that is mentioned in the question is a Ceasar's Cipher, a classic algorithm that gets mentioned in the very first lecture of almost all introductions to cryptography, only to show that it can be easily defeated by a frequency analysis. Another example is that naively composing building blocks / algorithms doesn't necessarily result in secure constructions, even if the algorithms themselves are: e.g. what is secure: sign-then-encrypt or encrypt-then-sign?

There are many Cryptography 101 courses on the net, both in textbook, tutorials, and online lecture form. Which one you select is a matter of personal taste. One popular entry point is Prof. Paar's series of 24 lectures on Basic Applied Cryptography (for total beginners), which is also available on YouTube, but feel free to explore other introductions. If you prefer to learn from books, and since you want to use Python, Practical Cryptography for Developers by Svetlin Nakov is a free book-in-progress that may be useful.

No matter what Cryptography 101 course you take, just follow along by doing the exercises and examples in Python, using a crypto module. Of course, implementing such simple algorithms as Ceasar's Cipher doesn't require such a module: this can be done with Python itself and no external module. But as soon as you dive deeper in applied cryptography, crypto modules will become very handy, since implementing current cryptographic algorithms "by hand" can be rather complicated for beginners and experienced users alike, and moreover, it is best practice to use battle-tested and peer-reviewed cryptographic libraries and modules, instead of rolling your own implementations, no matter how instructive that would be.

The takeaway from this answer is that it doesn't matter all that much, which specific tools and languages you use to implement cryptography: you need to get acquainted with the crypto basics first, learn about best practices, don't try to get fancy and don't roll your own solutions. Always keep in mind that errors in the design and use of cryptographic constructions can be subtle, even to experts. Stick to well-used design patterns that have been vetted by the cryptography community. Once you understand the basics, transferring your knowledge to specific programming languages and tools will be almost automatic.