CODEWITHSUNDEEP
pythonpython-3.x
magicsword
magicsword

Reputation: 1289

read a text file line by line and check for a substring on 2 of the lines

I want to read a text file and check for strings

with open(my_file,'r') as f:
    for line in f:
        if 'text1' in line:
            f.next()
            if 'text2' in line:
                # do some processing

I want to first find the text 'text1' at the beginning of the line then if found I want to check the next line for 'text2' if found then I will do some other processing. Appears that the f.next() is not moving to the next line.

Upvotes: 5

Views: 1715

Answers (3)

tdelaney
tdelaney

Reputation: 77337

Use the for loop iteration and use a state variable to decide whether you are in the "test1" case. That way you don't have to juggle a separate StopIteration exception.

text1_line = ""

with open(my_file) as f:
    for line in f:
        if text1_line and 'text2' in line:
            # do some processing
            text1_line = ""
            continue
        text1_line = line if 'text1' in line else ""

Upvotes: 3

blhsing
blhsing

Reputation: 106455

The variable line does not magically get updated when you call f.next() (or next(f) in Python 3). You would instead have to assign the line returned by next to a variable and test against it:

with open(my_file,'r') as f:
    for line in f:
        if 'text1' in line:
            try:
                next_line = next(f)
            except StopIteration:
                break # in case we are already at the end of file
            if 'text2' in next_line:
                # do some processing with line and next_line

If there can be cases where there are two consecutive lines with text1 followed by a line of text2, however, calling next(f) would make it consume the second line of text1, making it unable to match text1 in the next iteration. To remedy that, you can use the pairwise recipe in the itertools documentation to iterate 2 lines at a time over the file instead:

import itertools

def pairwise(iterable):
    a, b = itertools.tee(iterable)
    next(b, None)
    return zip(a, b)

with open(my_file,'r') as f:
    for line, next_line in pairwise(f):
        if 'text1' in line and 'text2' in next_line:
            # do some processing with line and next_line

Upvotes: 3

U13-Forward
U13-Forward

Reputation: 71570

Try with enumerate and readlines:

with open(my_file,'r') as f:
    lines = f.readlines() + ['']
    for idx, line in enumerate(lines):
        if 'text1' in line:
            if 'text2' in lines[idx + 1]:
                # do some processing

Upvotes: 2

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