In Python, searching for line number in file only works once, then only returns 0

Question

I am trying to use Python to edit Abaqus input files, and I need to find which lines have certain headers (e.g. "*Nodes," "*Elements"). I have code as follows that works correctly:

headerline = 0
for line in input.readlines():
    if line.lower().startswith('*node'):
        break 
    headerline = headerline + 1

print(headerline)

This prints the correct value, which in this case is 8. However, if I run this back to back, or try to search for another header, it always prints zero for the next header.

headerline = 0
for line in input.readlines():
    if line.lower().startswith('*node'):
        break 
    headerline = headerline + 1

print(headerline)

headerline = 0
for line in input.readlines():
    if line.lower().startswith('*node'):
        break 
    headerline = headerline + 1

print(headerline)

headerline = 0
for line in input.readlines():
    if line.lower().startswith('*node'):
        break 
    headerline = headerline + 1

print(headerline)

headerline2 = 0
for line in input.readlines():
    if line.lower().startswith('*element'):
        break 
    headerline2 = headerline2 + 1

print(headerline2)

Both of those examples print out the correct value for the first headerline, but then give 0 for the next one, even if I'm literally doing the exact same thing.

What am I missing here that is permanently setting it to zero?

Answer 1

You should close the file if you want to use readlines() again.

headerline = 0
for line in input.readlines():
    if line.lower().startswith('*node'):
        break 
    headerline = headerline + 1

print(headerline)

input.close()
input=open(...)

headerline2 = 0
for line in input.readlines():
    if line.lower().startswith('*element'):
        break 
    headerline2 = headerline2 + 1

print(headerline2)

Answer 2

as far as i know, it automatically closes the file after it has finished reading

Once you've run your code like this it's okay:

input = open(.........)
headerline = 0
for line in input.readlines():
    if line.lower().startswith('*node'):
        break 
    headerline = headerline + 1

print(headerline)

later when you want to access the same file again, you will get the value of []

you can learn value, like this:

print(input.readlines())#output:[]

Returns 0 since you can't look for anything with such a value