Apply replacement script on file

subst.py

#!/bin/python3

### Read a replacement script from stdin
### Read a filename from commandline args
### Output to stdout

### Replacement script has the following form

# ----eofmarker
# line:col:line:col
# multiline replacement
# text foobar quux
# eofmarker
# ----anothereof
# line:col:line:col
# etc etc etc
# anothereof

### The eofmarker can by any string (without newlines)
### The final newline of the replacement text is stripped.
### line:col:line:col are the coordinates
###     of the begin and end of replacement

import sys
import functools

def eprint(msg):
    print(msg, file=sys.stderr)
    sys.exit(1)

scripts = []
cur = {}
cur['lines'] = []
status = 0
for nr,line in enumerate(sys.stdin.readlines(),start=1):
    line = line.rstrip()
    if (status == 0): # at beginning of new chunk
        if (line[0:4] == "----"):
            cur['eof'] = line[4:]
            status = 1
        else:
            eprint("Illegal start of chunk in replacement script at line: " + str(nr))
    elif (status == 1): # parsing line and column numbers
        try:
            [sl1,sc1,sl2,sc2] = line.split(':')
            (l1,c1,l2,c2) = (int(sl1),int(sc1),int(sl2),int(sc2))
            assert [sl1,sc1,sl2,sc2] == [str(l1),str(c1),str(l2),str(c2)]
            assert l1 < l2 or (l1 == l2 and c1 <= c2)
            cur['coords'] = (l1,c1,l2,c2)
        except:
            eprint("Illegal location line in replacement script at line: " + str(nr))
        status = 2
    elif (status == 2):
        if (line == cur['eof']):
            if len(cur['lines']) > 0: # drop newline of final replacement string
                cur['lines'][-1] = cur['lines'][-1][:-1]
            scripts += [cur]
            cur = {}
            cur['lines'] = []
            status = 0
        else:
            cur['lines'] += line + '\n'
    else: # shouldn't happen
        eprint("Unreachable code was reached")

def subst(text,script):
    (l1,c1,l2,c2) = script['coords']
    lines = script['lines']
    for nr,line in enumerate(text,start=1):
        if nr < l1:
            yield line
        elif nr == l1:
            if c1 > len(line):
                eprint("Line is too short. Line number: " + str(nr))
            if len(lines) > 0:
                lines[0] = line[0:c1] + lines[0]
            else:
                lines = [line[0:c1]]
        if nr == l2:
            if c2 > len(line):
                eprint("Line is too short. Line number: " + str(nr))
            lines[-1] += line[c2:]
            for l in lines:
                yield l
        elif nr > l2:
            yield line
    if nr < l1:
        eprint("Replacement failed. File is too short.")

with open(sys.argv[1],"r+") as f:
    for line in functools.reduce(subst,reversed(scripts),f.readlines()):
        print(line,end='')