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Fix #387: Clean up backend type annotations page. #419

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Nov 19, 2024
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Fix #387: Clean up backend type annotations page. #419

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13 changes: 3 additions & 10 deletions Backend-Type-Annotations.md
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Type Annotations are a new feature added in [PEP 484](https://www.python.org/dev/peps/pep-0484/) that allow for adding type hints to variables. They give information about types of variables to someone who is reading the code. This brings a sense of statically-typed control to the dynamically typed Python. Though Python ignores these type hints during code execution, third-party libraries can be used to statically type-check the codebase.


## When to add type annotations
We use mypy to type check the backend part of our codebase. A file becomes eligible for type checking if it falls under one of the following 3 cases:
1. If a **new file** is being added, it must be fully type annotated and hence type checked.
2. If an **already type annotated file** is being updated, then the updated code must also have type annotations.
3. If you are adding type annotations to a currently **non type-annotated file**, then make sure to fully type annotate it, and remove it from the mypy denylist.

### Mypy Denylist - Important Note!
Currently we are in the process of adding type annotations the whole codebase. We maintain a denylist called `NOT_FULLY_COVERED_FILES` inside `scripts/run_mypy_checks` - it holds the list of files/folders for which we want to skip mypy type checking. When you add/modify files, make sure to **remove the file(s) eligible for type checking from this denylist if you have fully type annotated them**, otherwise it won't be considered for type checking, and the file might be even left with incorrect annotations.
To type-check the backend part of our codebase, we use mypy. **All backend files** should include type annotations.

## Running MyPy check script

Expand All @@ -29,6 +21,7 @@ make run_tests.mypy
```

This runs the type checks on all the type annotated files in the codebase.

2. Python:
```
python -m scripts.run_mypy_checks --files path/file1.py path/file2.py
Expand All @@ -46,7 +39,7 @@ make run_tests.mypy PYTHON_ARGS="--files path/file1.py path/file2.py"

## Adding type annotations

Once you've figured whether or not to add type annotations to a file, follow these steps. Here the method is simply to figure out types of the variables and mention them according to the syntax.
To add type annotations to a file, figure out the types of the variables and mention them according to the syntax.

### Steps to add type annotations to a file:

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