I'm trying to edit an MSA (Multiple Sequence Alignment) file generated by ClustalW, to trim sequences before the consensus one, using BioPython. 
*xxx refers to other bases not relevant here*

Here's the example I/O :

INPUT

    ITS_primer_fw               --------------------------------CGCGTCCACTMTCCAGTT
    RBL67ITS_full_sequence      CCACCCCAACAAGGGCGGCCACGCGGTCCGCTCGCGTCCACTCTCCAGTTxxxxxxxxxxxxxxxx
    PRL2010                     ACACCCCCGAAAGGGCGTCC------CCTGCTCGCGTCCACTATCCAGTTxxxxxxxxxxxxxxxx
    BBF32_3                     ACACACCCACAAGGGCGAGCAGGCG----GCTCGCGTCCACTATCCAGTTxxxxxxxxxxxxxx
    BBFCG32                     CAACACCACACCGGGCGAGCGGG-------CTCGCGTCCACTGTCGAGTTxxxxxxxxxxxxxxxx

EXPECTED OUTPUT

    ITS_primer_fw               CGCGTCCACTMTCCAGTT
    RBL67ITS_full_sequence      CGCGTCCACTCTCCAGTTxxxxxxxxxxxxxxxxxxxx
    PRL2010                     CGCGTCCACTATCCAGTTxxxxxxxxxxxxxxxxxxxxx
    BBF32_3                     CGCGTCCACTATCCAGTTxxxxxxxxxxxxxxxxxxx
    BBFCG32                     CGCGTCCACTGTCGAGTTxxxxxxxxxxxxxxxxxxxx

The documented code for `AlignIO` describes just a way to extract sequences by treating the alignment as an *array*. In this example 

    align = AlignIO.read(input_file, "clustal")
    sub_alignment = align[:,20:]
I was able to extract a subalignment made by all the sequences (:) starting from the 20th nucleotide. I'm looking for a way to replace the `20` in the example with the position of the first nucleotide of the consensus sequence. 

Any answers including some cline software to trim easly as requested are well accepted. Will be great if python coded or for UNIX.
This should work. Simply finds the index of the first column that doesn't contain a gap character.

    import sys
    from Bio import AlignIO
    
    # Usage: python scriptname.py alignment format
    
    aln = AlignIO.read(sys.argv[1], sys.argv[2])
    
    for col in range(aln.get_alignment_length()):
        if not "-" in aln[:, col]:
            position = col
            print("First full column is {}".format(col))
            break
    
    print("New alignment:")
    print(aln[:, position:])