Efficiently add multiple lines to file by index

Question

Given an input file data.dat like this one:

# Some comment
# more comments
#
45.78
# aaa
0.056
0.67
# aaa
345.
0.78
99.
2.34
# aaa
65.7
0.9

I need to add different comments above each line that starts with an "# aaa" so it looks like this:

# Some comment
# more comments
#
45.78
# cmmt1
# aaa
0.056
0.67
# another cmmt
# aaa
345.
0.78
99.
2.34
# last one
# aaa
65.7
0.9

I know a priori the number of "# aaa" comments present in the data.dat file, but not their positions.

I have a way to do it (see code below) but it is quite complicated and not at all efficient. I need to apply this code to hundreds of large files, so I'm looking for an efficient way to do this.

---

# Read file
with open("data.dat", mode="r") as f:
    data = f.readlines()

# Indexes of "# aaa" comments
idx = []
for i, line in enumerate(data):
    if line.startswith("# aaa"):
        idx.append(i)

# Insert new comments in their proper positions
add_data = ["# cmmt1\n", "# another cmmt\n", "# last one\n"]
for i, j in enumerate(idx):
    data.insert(j + i, add_data[i])

# Write final data to file
with open("data_final.dat", mode="w") as f:
    for item in data:
        f.write("{}".format(item))

Answer 1

I didn't do any benchmarks, but re.sub could be faster - just load the text file as whole, execute re.sub and write it out:

data = '''# Some comment
# more comments
#
45.78
# aaa
0.056
0.67
# aaa
345.
0.78
99.
2.34
# aaa
65.7
0.9'''

import re

def fn():
    add_data = ["# cmmt1\n", "# another cmmt\n", "# last one\n"]
    for d in add_data:
        yield d

out = re.sub(r'^# aaa', lambda r, f=fn(): next(f) + r.group(0), data, flags=re.MULTILINE)
print(out)

Prints:

# Some comment
# more comments
#
45.78
# cmmt1
# aaa
0.056
0.67
# another cmmt
# aaa
345.
0.78
99.
2.34
# last one
# aaa
65.7
0.9

With files input/output:

import re

def fn():
    add_data = ["# cmmt1\n", "# another cmmt\n", "# last one\n"]
    for d in add_data:
        yield d

with open('data.dat', 'r') as f_in, \
    open('data.out', 'w') as f_out:
    f_out.write(re.sub(r'^# aaa', lambda r, f=fn(): next(f) + r.group(0), f_in.read(), flags=re.MULTILINE))

Version 2:

import re

def fn():
    add_data = ["# cmmt1\n", "# another cmmt\n", "# last one\n"]
    add_data = [s + '#aaa' for s in add_data]
    for d in add_data:
        yield d

with open('data.dat', 'r') as f_in, \
    open('data.out', 'w') as f_out:
    f_out.write(re.sub(r'^# aaa', lambda r, f=fn(): next(f), f_in.read(), flags=re.MULTILINE))

Answer 2

According to Jan-Philip Gehrcke's response here, you should reduce the number of write calls.

To do so you could maybe simply change :

with open("data_final.dat", mode="w") as f:
    for item in data:
        f.write("{}".format(item))

to :

with open("data_final.dat", mode="w") as f:
    f.write("".join(data))

Answer 3

When I need to change data in a text file, I try to read with one handle and immediately write with a second one.

def add_comments(input_file_name, output_file_name, list_of_comments):
    comments = iter(list_of_comments)  # or itertools.cycle(list_of_comments)
    with open(input_file_name) as fin, open(output_file_name, 'w') as fout:
        for line in fin:
            if line.startswith("# aaa"):
                fout.write(next(comments))
            fout.write(line)

For your example code, if would be called as:

add_comments("data.dat", "final_data.dat", ["# cmmt1\n", "# another cmmt\n", "# last one\n"])