Python write function output to file

Question

I'm sorry if this is silly question but I have no much Python experience

I have function for comparing files

def compare_files(file1, file2):
    fname1 = file1
    fname2 = file2

    # Open file for reading in text mode (default mode)
    f1 = open(fname1)
    f2 = open(fname2)
    # Print confirmation
    #print("-----------------------------------")
    #print("Comparing files ", " > " + fname1, " < " +fname2, sep='\n')
    #print("-----------------------------------")

    # Read the first line from the files
    f1_line = f1.readline()
    f2_line = f2.readline()

    # Initialize counter for line number
    line_no = 1

    # Loop if either file1 or file2 has not reached EOF

    while f1_line != '' or f2_line != '':

       # Strip the leading whitespaces
      f1_line = f1_line.rstrip()
      f2_line = f2_line.rstrip()

      # Compare the lines from both file
      if f1_line != f2_line:

         ########## If a line does not exist on file2 then mark the output with + sign
        if f2_line == '' and f1_line != '':
            print ("Line added:Line-%d" % line_no + "-"+ f1_line)
         #otherwise output the line on file1 and mark it with > sign
        elif f1_line != '':
            print ("Line changed:Line-%d" % line_no + "-"+ f1_line)


        ########### If a line does not exist on file1 then mark the output with + sign
        if f1_line == '' and f2_line != '':
            print ("Line removed:Line-%d" % line_no + "-"+ f1_line)
          # otherwise output the line on file2 and mark it with < sign
         #elif f2_line != '':
            #print("<", "Line-%d" %  line_no, f2_line)

         # Print a blank line
         #print()

    #Read the next line from the file
      f1_line = f1.readline()
      f2_line = f2.readline()
      #Increment line counter
      line_no += 1

    # Close the files
    f1.close()
    f2.close()

I want to print function output to a text file

result=compare_files("1.txt", "2.txt")

print (result)
Line changed:Line-1-aaaaa
Line added:Line-2-sss
None

i tried following:

f = open('changes.txt', 'w')

f.write(str(result))

f.close

but only None is printed to changes.txt

I'm using "workaround" sys.stdout but wonder is there any other way instead of redirecting print output.

If in function output I specify return instead of print then I'm getting only first output line (Line changed:Line-1-aaaaa) to changes.txt

Answer 1

Your 'compare_files' function does not return anything and therefore nothing is written to the file. Make the function ' return ' something and it should work.

Answer 2

Your function isn't returning any thing so you are printing 'None'. If you want all the print to go to a file instead of stdout like it does by default you can chane each print statement like you did to the return value.

Or you can use redirection for the whole program like done in here .

Answer 3

Because you are not returning anything by default the function returns None so that is reflected in your changes.txt file. you can create a variable that stores the output that you wanted and returns it.

def compare_files(file1, file2):
    fname1 = file1
    fname2 = file2

    # Open file for reading in text mode (default mode)
    f1 = open(fname1)
    f2 = open(fname2)

    output_string = ""

    # Print confirmation
    # print("-----------------------------------")
    # print("Comparing files ", " > " + fname1, " < " +fname2, sep='\n')
    # print("-----------------------------------")

    # Read the first line from the files
    f1_line = f1.readline()
    f2_line = f2.readline()

    # Initialize counter for line number
    line_no = 1

    # Loop if either file1 or file2 has not reached EOF

    while f1_line != '' or f2_line != '':

        # Strip the leading whitespaces
        f1_line = f1_line.rstrip()
        f2_line = f2_line.rstrip()

        # Compare the lines from both file
        if f1_line != f2_line:

            ########## If a line does not exist on file2 then mark the output with + sign
            if f2_line == '' and f1_line != '':
                print("Line added:Line-%d" % line_no + "-" + f1_line)
                output_string += "Line added:Line-%d" % line_no + "-" + f1_line + "\n"
            # otherwise output the line on file1 and mark it with > sign
            elif f1_line != '':
                print("Line changed:Line-%d" % line_no + "-" + f1_line)
                output_string += "Line changed:Line-%d" % line_no + "-" + f1_line +"\n"

            ########### If a line does not exist on file1 then mark the output with + sign
            if f1_line == '' and f2_line != '':
                print("Line removed:Line-%d" % line_no + "-" + f1_line)
                output_string += "Line removed:Line-%d" % line_no + "-" + f1_line +"\n"
            # otherwise output the line on file2 and mark it with < sign
            # elif f2_line != '':
        # print("<", "Line-%d" %  line_no, f2_line)

        # Print a blank line
        # print()

        # Read the next line from the file
        f1_line = f1.readline()
        f2_line = f2.readline()
        # Increment line counter
        line_no += 1

    # Close the files
    f1.close()
    f2.close()
    return output_string

Answer 4

Your compare_files() just prints, but doesn't pass anything to its caller.

If you want to pass one item to the caller, you use return . The flow of your function ends there.

If you want to pass several items to the caller, you yield them. Using yield turns your function ito a generator function. Calling a generator function produces a generator object which can be iterated over.

Example:

def produce_strings():
    for i in ['a', 'b', 'c']:
        yield i + "x"

result = "\n".join(produce_strings())
print(result) # prints a line end separated string made of "ax", "bx" and "cx".