readlines / looping / matching string

Question

My goal is to take two text files, read them in, identify lines in file A that need to replace the same lines in file B and be saved as file C.

I am getting stuck on how to get the specific lines from file A. I need the whole line - my searchable string is essentially the first X characters, and always starts with a :.

from PyQt4.QtGui import *

searchable_strings = (':ABC',':123',':DEF')
entries_from_a = []

app = QApplication( sys.argv )

with open( unicode ( QFileDialog.getOpenFileName() )) as file_a:
    readlines_a = file_a.read()
file_a.closed

with open( unicode ( QFileDialog.getOpenFileName() )) as file_b:
    readlines_b = file_b.read()
file_b.closed
app.quit()

for line in readlines_a:
    for item in searchable_strings:
        if item in line:
            entries_from_a.append(line)

• is there a totally different way to do this?
• I am using the QApplication because I want the user to select the files each time.
• Currently, nothing is ever added to entries_from_a

• On a side note, every single time I run it, I have needed to quit Spyder and open a new window to continue due to this error

  QWidget: Must construct a QApplication before a QPaintDevice

Answer 1

The reason you not getting anything in entries_from_a is because for line in readlines_a reads one character at a time. And all your searchable_strings are more then 1 character long.

Here is an example:

echo "aaa\nbbb\n\ccc" > foo.txt

>>> with open('foo.txt') as f:
...   my_data = f.read()
... 
 >>> for i in my_data:
...   print i
... 
a
a
a
\
n
b
# and so on

So what you probably looking for is the way to iterate over each line.

Answer 2

Like @Vor said you are not iterating over lines in your for loop, but over the characters of a string. file_a.read() returns the whole file as one string. Using for instance readlines_a = file_a.readlines() would return a list of the lines of file_a . So would using a list comprehension, which is probably the faster option and better if your files are large. Then your for loop would iterate over lines as you intend.

with open( unicode ( QFileDialog.getOpenFileName() )) as file_a:
    readlines_a = file_a.readlines() # readlines() method

with open( unicode ( QFileDialog.getOpenFileName() )) as file_a:
    readlines_a = [line for line in file_a] # list comprehension

then your for loop

Alternatively:

with open('filefoo') as file_a:
    with open('filebar') as file_b:
        for line in file_a:
            etc.

Answer 3

You don't need to call the file's readlines method, since the file object is an iterable object.

with open( unicode ( QFileDialog.getOpenFileName() )) as f:
    readlines_a = [line for line in f]

print readlines_a