how to skip blocks of text when writing file in python

Question

Is it possible to use python to skip blocks of text when writing a file from another file?

For example lets say the input file is:

This is the file I would like to write this line
I would like to skip this line
and this one...
and this one...
and this one...
but I want to write this one
and this one...

How can I write a script that allows me to skip certain lines that differ in content and size which resumes writing the lines to another file once it recognizes a certain line?

My code reads through the lines, doesn't write duplicate lines and performs some operation on the line by using dictionaries and regex.

That entirely depends on why you want to skip those lines. There needs to be some codeable rule to select some lines and not others. Assuming you have that rule, this program becomes pretty simple really... — mgilson
– mgilson, Commented Jan 21, 2015 at 16:54

Hugh Bothwell · Accepted Answer · 2015-01-21 18:04:56Z

def is_wanted(line):
    #
    # You have to define this!
    #
    # return True to keep the line, or False to discard it

def copy_some_lines(infname, outfname, wanted_fn=is_wanted):
    with open(infname) as inf, open(outfname, "w") as outf:
        outf.writelines(line for line in inf if wanted_fn(line))

copy_some_lines("file_a.txt", "some_of_a.txt")

In order to extend this to multi-line blocks, you can implement a finite state machine like

enter image description here

which would turn into something like

class BlockState:
    GOOD_BLOCK = True
    BAD_BLOCK = False

    def __init__(self):
        self.state = self.GOOD_BLOCK

    def is_bad(self, line):
        # *** Implement this! ***
        # return True if line is bad

    def is_good(self, line):
        # *** Implement this! ***
        # return True if line is good

    def __call__(self, line):
        if self.state == self.GOOD_BLOCK:
            if self.is_bad(line):
                self.state = self.BAD_BLOCK
        else:
            if self.is_good(line):
                self.state = self.GOOD_BLOCK
        return self.state

then

copy_some_lines("file_a.txt", "some_of_a.txt", BlockState())

André Laszlo · Accepted Answer · 2015-01-21 16:55:32Z

2

Pseudo-code:

# Open input and output files, and declare the unwanted function
for line in file1:
    if unwanted(line):
        continue
    file2.write(line)
# Close files etc...

answered Jan 21, 2015 at 16:55

André Laszlo

15.6k3 gold badges68 silver badges83 bronze badges

Comments

Maroun · Accepted Answer · 2015-01-21 16:56:40Z

0

You can read the file line by line, and have control on each line you read:

with open(<your_file>, 'r') as lines:
    for line in lines:
        # skip this line
        # but not this one

Note that if you want to read all lines despite the content and only then manipulate it, you can:

with open(<your_file>) as fil:
    lines = fil.readlines()

answered Jan 21, 2015 at 16:56

Maroun

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Comments

urim · Accepted Answer · 2015-01-21 17:03:21Z

0

This should work:

SIZE_TO_SKIP = ?
CONTENT_TO_SKIP = "skip it"

with open("my/input/file") as input_file:
    with open("my/output/file",'w') as output_file:
        for line in input_file:
            if len(line)!=SIZE_TO_SKIP and line!=CONTENT_TO_SKIP:
                output_file.write(line)

answered Jan 21, 2015 at 17:03

urim

5914 silver badges15 bronze badges

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