[FIXED] how to skip blocks of text when writing file in python

Issue

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.

Solution

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

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())

Answered By - Hugh Bothwell

Answer Checked By - Clifford M. (PHPFixing Volunteer)