Removing lines in my file that contain a certain variable in python

My test.txt looks like

bear
goat
cat

what im trying to do is take the first line of it, which is bear and find and lines that contain it then delete them, the problem here is when I run my code all it does is delete all of the contents of my output file.

import linecache
must_delete = linecache.getline('Test.txt', 1)
with open('output.txt','r+') as f:
    data = ''.join([i for i in f if not i.lower().startswith(must_delete)])
    f.seek(0)                                                         
    f.write(data)                                                     
    f.truncate()

Solution

What you want is in-place editing, meaning read and write at the same time, line by line. Python has the fileinput module which offers this ability.

from __future__ import print_function
import linecache
import fileinput

must_delete = linecache.getline('Test.txt', 1)

for line in fileinput.input('output.txt', inplace=True):
    if line != must_delete:
        print(line, end='')

Notes

The calls to fileinput.input() includes the parameter inplace=True which specifies in-place editing
Within the with block, because of the in-place editing, the print() function (by magic) will print to the file, not your console.
We need to call print() with end='' to avoid extra line-ending char(s). Alternatively, we can omit the from __future__ ... line and do use the print statement like this (note the ending comma):
```
print line,
```

Update

If you want to detect the presence of the first line (e.g. 'bear') then there are two things more to do:

In previous code, I did not strip new line from must_delete, so it might looks like bear\n. Now we need to strip off the new line in order to test anywhere within the line
Instead of comparing the line with must_delete, we must do a partial string comparison: if must_delete in line:

Putting it all together:

from __future__ import print_function
import linecache
import fileinput

must_delete = linecache.getline('Test.txt', 1)
must_delete = must_delete.strip()  # Additional Task 1

for line in fileinput.input('output.txt', inplace=True):
    if must_delete not in line:  # Additional Task 2
        print(line, end='')

Update 2

from __future__ import print_function
import linecache
import fileinput

must_delete = linecache.getline('Test.txt', 1)
must_delete = must_delete.strip()
total_count = 0  # Total number of must_delete found in the file

for line in fileinput.input('output.txt', inplace=True):
    # How many times must_delete appears in this line
    count = line.count(must_delete)
    if count > 0:
        print(line, end='')
    total_count += count  # Update the running total

# total_count is now the times must_delete appears in the file
# It is not the number of deleted lines because a line might contains
# must_delete more than once