Write text expander in Python

[buran]

My question was about how to make a programm that run in background of OS and replace letters automatically when user it writes anywhere, for example, here, in this post.

nope, nothing of that kind in your original question :-)

Hi

I want to write simple program in Python that replace the certain sequence of letters with some unicode symbol. For example, "delta_low" -> "δ". User defines input and output.

I have no idea how to implement it. What the place to start?