replace a string within file using python

Question

I am getting an odd problem. when I am trying to replace a string in a file. The relevant line in the file is:

 lattice parameter A  [a.u.] 
    5.771452243459

and I am trying to replace it as:

   with open(newsys, "r+") as finp:
        for line in finp:
            # print(line)
            if line.startswith("lattice parameter A  [a.u.]"):
                line = next(finp)
                print(line)
                print(Alat)
                line.replace(line.strip(), str(Alat))
                print(line)

the last 3 print statement gives:

    5.771452243459  # string that will be replaced

6.63717007997785    #value of Alat
    5.771452243459  #the line after replace statement

What is going wrong here?

mbednarski · Accepted Answer · 2016-08-11 14:07:57Z

1

replace method does not modify existing string. Instead it is creating a new one. So in line

line.replace(line.strip(), str(Alat))

You are creating a completely new string and discards it (because not assign to any variable).

I would do something like:

   with open(newsys, "r+") as finp:
        with open('newfile', 'w') as fout:
            for line in finp:
                # print(line)
                if line.startswith("lattice parameter A  [a.u.]"):
                    line = next(finp)
                    print(line)
                    print(Alat)
                    line = line.replace(line.strip(), str(Alat))
                    print(line)
                fout.write(line)

edited Aug 11, 2016 at 14:07

answered Aug 11, 2016 at 13:16

mbednarski

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