Modifying specific contents of a text file in Python

October 8, 2022

I want to modify beta and n2 in Test.txt. However, f0.write() is completely rewriting all the contents. I present the current and expected outputs.

fo = open("Test.txt", "w") 
fo.write( "beta=1e-2")
fo.write( "n2=5.0")
fo.close()

The contents of Test.txt is

beta=1e-1
n1=30.0
n2=50.0

The current output is

beta=1e-2n2=5.0

The expected output is

beta=1e-2
n1=30.0
n2=5.0

>Solution :

First you need to read the contents of the file and store it on a dictionary, preferably using the with statement:

data = {}
with open('Test.txt') as f:
    for line in f:
        line = line.strip()
        # Ignore empty lines
        if not line:
            continue

        # Split every line on `=`
        key, value = line.split('=')
        data[key] = value

Then, you need to modify the read dictionary to the contents you want:

data['beta'] = 1e-2
data['n2'] = 5.0

Finally, open the file again on write mode and write back the dictionary:

with open('Text.txt', 'w') as f:
    for key, value in data.items():
        # Join keys and values using `=`
        print(key, value, sep='=', file=f)