Check if a string is ensembl gene id or not in python
Fixed it myself actually, not that hard. I found regular expression for ensembl ids here:

https://www.biostars.org/p/99142/

And used the re package in python:

import re

ensembl_regexp = 'ENS[A-Z]+[0-9]{11}'

is_ensembl = np.all([bool(re.match(ensembl_regexp, gene)) for gene in gene_symbols])
    if is_ensembl:
        gene_symbols = list(f['ensembl_ids']) 


Where gene_symbols is a numpy array that I need to check whether it has ensembl ids, and if yes, switch to using gene symbols which are going to be under ensembl_ids key due to a mistake of someone who made the hdf5 dataset.