Running a for loop in python

Question

I'm trying to generate 1000 new files from already existing 1000 files in a folder named as 'mdp' into a new folder 'mdb', changing couple of lines in each of the original 1000 files. I took some help from overflow and @bruno desthuilliers suggested me to use this particular code but it hasn't worked yet and it says dest.write(line) AttributeError: 'str' object has no attribute 'write'. I'm kind of new to programming. Could anyone please tell me what am I missing?

 import os
 sourcedir = "/home/abc/xyz/mdp/"
 destdir = "/home/abc/xyz/mdb/"
 for filename in os.listdir(sourcedir):
 if not filename.endswith(".mdp"):
 continue
 source = os.path.join(sourcedir, filename)
 dest = os.path.join(destdir, filename)
 fin = open("/home/abc/xyz/mdp/md_1.mdp")
 fout = open("/home/abc/xyz/mdb/md_1.mdp", "w")
 for line in source:
 line = line.replace('integrator = md', 'integrator = md-vv')
 line = line.replace('dt = 0.001', 'dt = 
 -0.001')
 dest.write(line)

• mlwn: are you sure about the indentation ??? this is very critical in python.. the for loop indentation doesn't seem to be correct
• Data_Is_Everything: you want to make use of the fin var instead of source

Answer 1

You messed up with path (which str) and file objects (which a readable/writable), so you've to fix a bit your code:

with open(source) as fin, open(dest, "w") as fout:
 for line in fin:
 fout.write(
 line.replace(
 'integrator = md',
 'integrator = md-vv',
 ).replace(
 'dt = 0.001',
 'dt = -0.001',
 )
 )

also use with to properly close opened files in case of runtime errors.

Answer 2

The problem is that dest is a string, not an instance of an open file. The code that I modified should work. Please give it a try and let me know how does it go!

import os
sourcedir = "/home/abc/xyz/mdp/"
destdir = "/home/abc/xyz/mdb/"
for filename in os.listdir(sourcedir):
 if not filename.endswith(".mdp"):
 continue
 # Gets the fullpath for the source and dest files
 source = os.path.join(sourcedir, filename)
 dest = os.path.join(destdir, filename)
 # Open the files in read and write mode
 fin = open(source,"r")
 fout = open(dest, "w")
 # Creates a list of the lines contained in the 'dest' file
 lines = fin.readlines()
 # Close the source file as is no longer needed
 fin.close()
 # Loop over each line to implement the modifications
 for line in lines:
 line = line.replace('integrator = md', 'integrator = md-vv')
 line = line.replace('dt = 0.001', 'dt = 
 -0.001')
 # Writes the replaced line into the 'dest' file
 fout.write(line)
 # Close the 'dest' file after finishing looping
 fout.close()

Also, the code that you posted has some indentation issues, I fixed them here.

• accidental_coder: Thanks @EnriqueBet. This is giving me a similar error. fin = source.readlines() AttributeError: 'str' object has no attribute 'readlines'
• accidental_coder: Still did not work, it created just one file which was md_863.mdp which was also blank. Am I supposed to actually state the source in the 7th line of this code in place of sourcedir?