Can a paper be rejected for only detailing an algorithm mathematically and not programmatically (via pseudocode)?
In theoretical computer science (I cannot speak for other fields) it is very common not to give pseudocode, but only a high-level description of the algorithm. Your paper should always explain your ideas as clearly as possible, and as an author it is your job to select the most suitable presentation. This does not necessarily need to be pseudocode. Painstakingly presenting every little implementation detail may not add much to the paper and instead distract the reader. If you think foregoing pseudocode results in a better paper, by all means do so (unless the journal you are submitting to specifically requires it).
If your paper presents an implementation in Python, I would expect the source code to be publicly available somewhere and referenced in the paper.