txt-file: read and append missing data

[Gribouillis]

You need to parse the input one way or another. Here is a simple parser. It reads the input lines and for each line it calls a method line_event(). Initially this method simply outputs the line to the output file. If it meets a line starting with .CURVE, it changes the method to line_event_curve_header() which outputs the lines until it meets a line ..NE. In the meanwhile it detects if it meets a line starting with ...A_Z or ...B_Z. After the last line of the header, if ...A_Z or ...B_Z has not been found it changes the method to line_event_curve_body() which appends 0.00 to the lines until it meets a line starting a new object.

Here is the (untested) code

import re

class Parser:
    object_pattern = re.compile(r'^[.](CURVE|POINT|TEXT)')
    
    def __init__(self, infile, outfile):
        self.infile = infile
        self.outfile = outfile
        self.line_event = self.line_event_base
        
    def run(self):
        for line in self.infile:
            self.line_event(line)
        self.outfile.flush()
        
    def line_event_base(self, line):
        if line.startswith('.CURVE'):
            self.start_curve(line)
        else:
            self.outfile.write(line)
            
    def line_event_curve_header(self, line):
        if line.startswith('...A_Z') or line.startswith('...B_Z'):
            self.seen_ab_z = True
            self.output.write(line)
        elif line.rstrip('\n') == '..NE':
            if self.seen_ab_z:
                self.line_event = self.line_event_base
                self.output.write(line)
            else:
                self.output.write('...A_Z 0.00\n...B_Z 0.00\n..NEZ\n')
                self.line_event = self.line_event_curve_body
        else:
            self.output.write(line)
            
    def line_event_curve_body(self, line):
        if self.object_pattern.match(line):
            if line.startswith('.CURVE'):
                self.start_curve(line)
            else:
                self.line_event = self.line_event_base
                self.outfile.write(line)
        else:
            self.outfile.write(line.rstrip('\n'))
            self.outfile.write(' 0.00\n')
                
    def start_curve(self, line):
        self.line_event = self.line_event_curve_header
        self.seen_ab_z = False
        self.line_event(line)

if __name__ == '__main__':
    with open('C:/Users/sufi/Desktop/info.txt','r') as infile,\
            open('C:/Users/sufi/Desktop/info_edit.txt','w') as outfile:
        Parser(infile, outfile).run()