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_posts/2024-06-10-PythonsPreprocessor.md

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+---
+title: Python's Preprocessor
+date: 2024-06-10T23:36:29+00:00
+categories: [Python]
+tags: [Python, Hackery]
+author: Che
+bokeh: true
+---
+
+Every now and the you hear outrageous claims such as "Python has no preprocessor". This is simply not true. In fact, Python has the best preprocessor of all languages - it quite literally allows us to do whatever we want, and a lot more. It's just a little tricky to (ab)use.
+
+# Python Source Code Encoding
+According to [PEP-0263](https://peps.python.org/pep-0263/#defining-the-encoding) it is possible to define a source code encoding by placing a magic comment in one of the first 2 lines. The following lines would all be interpreted as setting the encoding to `utf8`:
+```py
+# coding=utf8
+# -*- coding: utf8 -*-
+# vim: set fileencoding=utf8 :
+```
+
+To be precise, the line must match the regular expression `^[ \t\f]*#.*?coding[:=][ \t]*([-_.a-zA-Z0-9]+)`. Naturally we can use our own encodings, but their names must match `[-_.a-zA-Z0-9]+`.
+
+# Defining Custom Codecs
+https://docs.python.org/3/library/codecs.html
+
+# Extending Python
+## Unary increment and decrement
+https://github.com/dankeyy/incdec.py
+
+In Python the postfix operators `x++` can be written as `((x, x := x+1)[0])`, `x--` is therefore `((x, x := x-1)[0])`. Using the same expression we can write the prefix operators `++x` as `((x, x := x+1)[1])` and `--x` as `((x, x := x-1)[1])` respectively. This expression works by pulling out the respective element from a tuple of itself and itself updated.
+
+Other than that it's simply text replacement. 
+
+# Polyglotting
+
+Instead of expanding Python, why not teach the Python interpreter a few more tricks? After all there's all kinds of cool languages it could interpret!
+
+## C and C++
+The easiest way to smuggle the magic line into C and C++ sources is by defining a macro like
+```c
+#define CODEC "coding:pydong"
+```
+Great, we can now trigger the `pydong` decoder with a valid C or C++ source file. To actually get the Python interpreter to interpret this C or C++ code for us, we can use the excellent package `cppyy`. In essence `cppyy` uses `cling` under the hood to interpret our code and generates Python bindings for us to use it. 
+
+After our decoder is done with the input file, the output should look something like
+```py
+import cppyy
+
+# interpret the input source code
+cppyy.cppdef("<input source file content>")
+
+# find the main function
+from cppyy.gbl import main
+
+if __name__ == "__main__":
+  # call C/C++ main
+  main()
+```
+
+Now we can run `python foo.cpp` if `foo.cpp` begins with the magic line `#define CODEC "coding:pydong"` or similar.
+
+## Shell script
+Shell script comments start with `#`, hence we don't need to do anything special for the magic line.
+
+## CMake
+Just like Shell scripts, CMake uses `#` for comments.
+
+## PHP
+PHP allows `#` comments, for example for the shebang. 
+
+## Ruby
+Ruby uses `#` for single-line comments. This means just like PHP, we can simply use a comment for the magic line.