Member Avatar for pyprog

Assume I have a file of the following format:
a,1
b,2
c,3
d,4
Here is my code:

def junk(f):
    d1 = {}
    d2 = {}
    for line in f:
        columns = line.split(",")
        letters = columns[0]
        numbers = columns[1]
        d1[letters] = numbers
        d2[numbers] = letters
    return (d1, d2)
    
def something():  
    print d1
    print d2
    
if __name__ == "__main__":
    f = open("filename.txt")
    d1 = junk(f)[0]
    d2 = junk(f)[1]

Assume I want to call function something. It cannot print d1 and d2 unless I add them to the main block. As soon as I add d1 and d2 to the "main" block and call either of the two functions, both return two empty dictionaries. Why does this happen? How can I fix it? Please help! I've been trying to figure it out the whole day!

Edited by pyprog because: n/a
  • 3 Contributors
  • 3 Replies
  • 289 Views
  • 21 Hours Discussion Span
  • Latest Post Latest Post by vegaseat

Dani AI

Generated

The symptom (empty dicts) comes from reading the same file handle more than once: the first read consumes the file (the iterator goes to EOF), so a second call sees no lines and returns empty mappings. was on the right track asking whether the loader actually returns data; the usual bug is not the parser itself but how it's called. correctly noted the issue was solved elsewhere.

Fixes (choose one)

  • Call the parser once and unpack both dictionaries: do not invoke the reader twice on the same file object.
  • Rewind the file before a second read with f.seek(0).
  • Prefer reopening the file or (better) returning both dicts and passing them to other functions instead of relying on globals.

Example of a robust pattern:

def parse_file(fh):
    a_to_b = {}
    b_to_a = {}
    for raw in fh:
        line = raw.strip()
        if not line:
            continue
        key, val = line.split(',', 1)
        key = key.strip()
        val = val.strip()
        try:
            val = int(val)
        except ValueError:
            pass
        a_to_b[key] = val
        b_to_a[val] = key
    return a_to_b, b_to_a

with open('filename.txt') as fh:
    a_to_b, b_to_a = parse_file(fh)

def something(a_map, b_map):
    print(a_map)
    print(b_map)

something(a_to_b, b_to_a)

Extra tips

  • Use split(',', 1) to avoid breaking values that contain commas.
  • Always strip() lines to remove newlines and whitespace.
  • Avoid globals: pass the dicts into functions so the flow is explicit.
  • If you really must call the parser twice, call fh.seek(0) between calls or reopen the file.

Recommended Answers

All 3 Replies

Member Avatar for mn_kthompson

What is the load_airports function? Are you sure that it is returning something into your dictionaries?

Member Avatar for pyprog

What is the load_airports function? Are you sure that it is returning something into your dictionaries?

Sorry, copied a part of the other program. I fixed it.

Be a part of the DaniWeb community

We're a friendly, industry-focused community of developers, IT pros, digital marketers, and technology enthusiasts meeting, networking, learning, and sharing knowledge.