Infinite loop using iter method

Question

I am an amateur and am playing around with writing my own (very bad) compression tool, just for fun. The following code is used to read a text file and create a dictionary of the indexes of every character in the file. I'm trying to read the file in 1K chuncks, just for the hell of it but for some reason I get an infinite loop. I'm guessing I've misunderstood something in the "Iter" method.

code:

def dictify(myFile):
    compDict = {}
    count = 0
    with open(myFile, 'r') as f:
        for chunk in iter(f.read, 1024):
            for ch in chunk:
                if ch in compDict:
                    compDict[ch].append(count)
                else:
                    compDict[ch] = []
                    compDict[ch].append(count)
                count += 1
            print(compDict)
    print(compDict)

dictify('test.txt')

the print statement was for debugging purposes and I left it in because it will make it clear to whomever runs the code where the inf. loop is. Also - the txt file can be anything. Mine just says "I am the walrus"

any ideas what I'm doing wrong? Thanks!

Answer 1

this is not how iter works.

your example is given in the doc as:

from functools import partial

with open('mydata.db', 'rb') as f:
    for block in iter(partial(f.read, 64), b''):
        process_block(block)

if you use iter with 2 arguments, the first must be a callable and the second a sentinel; i.e. something to look for when iter needs to terminate.

in your case the second argument is an integer (1024); f.read returns a string; so it will never terminate.

if you read your file in text mode (as opposed to binary) you need to make the following changes (i also adapted your block size):

with open('mydata.db', 'r') as f:
    for block in iter(partial(f.read, 1024), ''):
        process_block(block)