Reading Input From A File In Python 3.x

Say you are reading input from a file structured like so P3 400 200 255 255 255 255 255 0 0 255 0 0 etc... But you want to account for any mistakes that may come from the input fi

Solution 1:

Here is one way to go about it, using csv module:

import csv
first_four = []
all_of_the_tokens = []
first_four_processed = Falsewithopen('token') as token_file:
    csv_reader = csv.reader(token_file, delimiter=' ')
    for row in csv_reader:
        all_of_the_tokens.extend(row)
        ifnot first_four_processed:
            first_four.extend(row)
        iflen(first_four) >= 4andnot first_four_processed:
            first_four_processed = True
            first_four = first_four[:4]
token_file.close()

rest_of_the_tokens = all_of_the_tokens[4:]

for i inrange(0, len(rest_of_the_tokens), 3):
    print rest_of_the_tokens[i:i+3]

Solution 2:

If your file consists of groups of three values (after the first P3 item) and you cannot rely upon the line breaks to have them grouped properly, I suggest reading the file as a single string and doing the splitting and grouping yourself. Here's a straight-forward way:

withopen(filename) as f:
    text = f.read()    # get the file contents as a single string

tokens = text.split()  # splits the big string on any whitespace, returning a list
it = iter(tokens)      # start an iterator over the list
prefix = next(it)      # grab the "P3" token off the front
triples = list(zip(it, it it))  # make a list of 3-tuples from the rest of the tokens

Using zip on multiple references to the same iterator is the key trick here. If you needed to handle other group sizes with the same code, you could use zip(*[it]*grouplen).

Note that this will discard any left-over values at the end of the file if they don't form a group of three. If you need to handle that situation differently, I suggest using zip_longest from the itertools module, rather than the regular zip function. (See the grouper recipe in the itertools documentation.)