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

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)

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