如何更改打印以在python中的for循环中返回？

Question

The following code is doing most of what I want...

All I need is for that print to actually be a return so that I can dump the data into another txt file I'm writing ( f2 ).

(Also, the spacing obtained with print letters is not what I want but I figure I'll deal with it later.)

Every time I replace print with return it just stops reading after the first line of the initial text file ( f1 ).

def DNA2Prot(f1, f2="translated_fasta.txt"):
    fin = open(f1, 'r')
    for letters in fin:
        if letters[0] != ">":
            seqs = letters
            codons = [ ]
            protein = ''
            for i in range(0, len(seqs), 3):
                try:
                    codon = seqs[i:i+3]
                    codons = codon_table[codon]
                    protein = protein+codons
                except KeyError:
                    protein += ""
            print protein
        else:
            print letters
    fin.close()

Answer 1

Use yield instead and treat your function as a generator. This way the caller can do what he/she pleases with all of the proteins the DNA2Prot function generates and read from the file until the entire file is read.

def DNA2Prot(f1, f2='translated_fasta.txt'):
    # prefer using `with` to `open` and `close`
    with open(f1, 'r') as fin:
        for letters in fin: 
            if letters[0] != '>':
                seqs = letters
                codons = [ ]
                protein = ''
                for i in range(0, len(seqs), 3):
                    # no need for a try catch, because we can use `get`
                    # get will return None by default if the 
                    # specified `codon` does not appear in 
                    # `codon_table`
                    codon = seqs[i:i + 3]       
                    codons = codon_table.get(codon)
                    if codons:
                        protein += codons
                yield protein
            else:
                yield letters        

Now you have to treat the DNA2Prot function as an Iterator :

with open('/path/to/outfile', 'w') as f:
    for protein in DNA2Prot(f1):
        # do something with protein
        print protein

Answer 2

First things first. When you use the return statement you are telling your code to break out(ie leave) from the point where the return statement is located. This means that your code will start reading from fin, move on to the second for and as soon as it is done with it (read all the letters of the line) it will reach you return statement and break out from the DNA2prot function.

Now, there are two things you can do to when it comes to files. First is use the print function to redirect your output to a file (not recommended) or properly open the files and write into them.

With regards to the first solution (and assuming you are using python 2.7) you can simply do:

from __future__ import print_function

and when you want to use your print statement just write:

print(protein, file = fin).

However, if I were you I would go for a more elegant and clean solution that doesn't rely on unnecessary imports:

def DNA2Prot(f1, f2="translated_fasta.txt"):
with open (f1, 'r+') as fin, open(f2, 'w+') as fin2: #Using the "with-open" statement you don't need to close the file object
    for letters in fin:
        if letters[0]!=">":
            seqs=letters
            codons=[ ]
            protein=''
            for i in range(0,len(seqs),3):
                try:
                    codon=seqs[i:i+3]
                    codons=codon_table[codon]
                    protein=protein+codons
                except KeyError:
                     protein+=""
            f2.write(protein) # Write your data to the second file             

        else:
            f2.write(letters)