Gain insights into Python 3's new syntax, data types, strings, bytes, and iterators through interactive coding samples. Upgrade your Python skills with hands-on, real-time code modifications.

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Learn about the new syntax, data types, strings, bytes, iterators and much more with interactive coding samples. Execute code using interactive playgrounds, modify the code to see your changes in action and get yourself upgraded to the newest version of Python that is here to stay for a while.

This course is derived from Dive into Python 3 by Mark Pilgrim. See License for more details.
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Python 3: An interactive deep dive

Now that the `from_roman()` function works properly with good input, it's time to fit in the last piece of the puzzle: making it work properly with bad input. That means finding a way to look at a string and determine if it's a valid Roman numeral. This is inherently more difficult than [validating numeric input](https://www.educative.io/courses/python-3-an-interactive-deep-dive/more-halting-more-fire/) in the `to_roman()` function, but you have a powerful tool at your disposal: regular expressions. (If you’re not familiar with regular expressions, now would be a good time to read the regular expressions chapter.)

As you saw in [Case Study: Roman Numerals](https://www.educative.io/courses/python-3-an-interactive-deep-dive/case-study-roman-numerals/), there are several simple rules for constructing a Roman numeral, using the letters `M`, `D`, `C`, `L`, `X`, `V`, and `I`. Let's review the rules:

* Sometimes characters are additive. `I` is `1`, `II` is `2`, and `III` is `3`. `VI` is 6 (literally, “5 and 1”), `VII` is `7`, and `VIII` is `8`.
* The tens characters (`I`, `X`, `C`, and `M`) can be repeated up to three times. At `4`, you need to subtract from the next highest fives character. You can't represent `4` as `IIII`; instead, it is represented as `IV` (“`1` less than `5`”). `40` is written as `XL` (“`10` less than `50`”), `41` as `XLI`, `42` as `XLII`, `43` as `XLIII`, and then `44` as `XLIV` (“`10` less than `50`, then `1` less than `5`”).
* Sometimes characters are… the opposite of additive. By putting certain characters before others, you subtract from the final value. For example, at `9`, you need to subtract from the next highest tens character: `8` is `VIII`, but `9` is `IX` (“1 less than 10”), not `VIIII` (since the I character can not be repeated four times). `90` is `XC`, `900` is `CM`.
* The fives characters can not be repeated. `10` is always represented as `X`, never as `VV`. `100` is always `C`, never `LL`.
* Roman numerals are read left to right, so the order of characters matters very much. `DC` is `600`; `CD` is a completely different number (`400`, “`100` less than `500`”). `CI` is `101`; `IC` is not even a valid Roman numeral (because you can't subtract `1` directly from `100`; you would need to write it as `XCIX`, “`10` less than `100`, then 1 less than `10`”).

Thus, one useful test would be to ensure that the `from_roman()` function should fail when you pass it a string with too many repeated numerals. How many is “too many” depends on the numeral.

Your First Python Program

Native Datatypes

Comprehensions

Strings

Regular Expressions

Closures & Generators

Classes & Iterators

Advanced Iterators

Unit Testing

Refactoring

Files

XML

Serializing Python Objects

HTTP Web Services

Case Study: Porting chardet to Python 3

Packaging Python Libraries

Appendix : Where To Go From Here

More Bad Input