How can I check whether a given file is FASTA?

I am designing a code that requires a .fasta file to be input at one of the early stages. Right now, I am validating the input using this function:

def file_validation(fasta):
    while True:
        try:
            file_name= str(raw_input(fasta))
        except IOError:
            print("Please give the name of the fasta file that exists in the folder!")
            continue

        if not(file_name.endswith(".fasta")):
            print("Please give the name of the file with the .fasta extension!")
        else:
            break
    return file_name

Now, although this function works fine, there is still some room for error in the sense that a user could potentially maybe input a file that, while having a file name that ends with .fasta, could have some non-.fasta content inside. What could I do to prevent this and let the user know that his/her .fasta file is corrupted?

Solution

Why not just parse the file as if it were FASTA and see whether it breaks?

Using biopython, which silently fails by returning an empty generator on non-FASTA files:

from Bio import SeqIO

my_file = "example.csv"  # Obviously not FASTA

def is_fasta(filename):
    with open(filename, "r") as handle:
        fasta = SeqIO.parse(handle, "fasta")
        return any(fasta)  # False when `fasta` is empty, i.e. wasn't a FASTA file

is_fasta(my_file)
# False