在函数中使用readline（）来读取日志文件不会进行迭代

Question

In the code below readline() will not increment. I've tried using a value, no value and variable in readline() . When not using a value I don't close the file so that it will iterate but that and the other attempts have not worked.

What happens is just the first byte is displayed over and over again.

If I don't use a function and just place the code in the while loop (without 'line' variable in readline() ) it works as expected. It will go through the log file and print out the different hex numbers.

i=0
x=1

def mFinder(line):
    rgps=open('c:/code/gps.log', 'r')
    varr=rgps.readline(line)
    varr=varr[12:14].rstrip()
    rgps.close()
    return varr


while x<900:
    val=mFinder(i)
    i+=1
    x+=1
    print val
    print 'this should change'

Answer 1

It appears you have misunderstood what file.readline() does. Passing in an argument does not tell the method to read a specific numbered line.

The documentation tells you what happens instead:

file.readline([size])
Read one entire line from the file. A trailing newline character is kept in the string (but may be absent when a file ends with an incomplete line). If the size argument is present and non-negative, it is a maximum byte count (including the trailing newline) and an incomplete line may be returned.

Bold emphasis mine, you are passing in a maximum byte count and rgps.readline(1) reads a single byte, not the first line.

You need to keep a reference to the file object around until you are done with it, and repeatedly call readline() on it to get successive lines. You can pass the file object to a function call:

def finder(fileobj):
    line = fileobj.readline()
    return line[12:14].rstrip()

with open('c:/code/gps.log') as rgps:
    x = 0
    while x < 900:
        section = finder(rgps)
        print section
        # do stuff
        x += 1

You can also loop over files directly , because they are iterators:

for line in openfilobject:

or use the next() function to get a next line, as long as you don't mix .readline() calls and iteration (including next() ). If you combine this witha generator function , you can leave the file object entirely to a separate function that will read lines and produce sections until you are done:

def read_sections():
    with open('c:/code/gps.log') as rgps:
        for line in rgps:
            yield line[12:14].rstrip()

for section in read_sections():
    # do something with `section`.