replacing certain substrings in lines of a text file

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Similar questions to this question have been answered before.
I want to replace particular substrings in lines of a text file. Based on the answers to similar questions, I came up with the following.

with open('grammars.txt', 'r') as file3:
    # read a list of lines into data
    data = file3.readlines()

mylist = []
# Assuming that you have loaded data into a lines variable. 

  
for line in data:
    if 'n' in line:
        line.strip('n')
        print('hello')
    if '|' in line:
       line.replace('|','||')
       print('hello')
    if '&' in line:
      line.replace('&','&&')  
      print('hello')
    mylist.append(line)  
print(mylist)

The problem is that when I run this code no changes happen to the strings in data. I tried to use "" instead of '' and searched for possible reasons why this code does not change anything. I made sure that at least in one line of the text file, the condition of at least one of the ifs is met by printing hello.
I would appreciate it if you could help me with this.
Here is the text file I am using.

NOTE: Once I store the text file lines using readlines(), n appears at the end of each string which I need to remove.

N0>|,N1,N2&
N1>x0
N2>x1&
-----------------------
N0>U,N1,N2
N1>x1&
N2>x0
-----------------------
N0>U,N1,N3|
N1>G,N2
N2>x1
N3>x0
-----------------------
N0>->,N1,N3
N1>!,N2
N2>x1
N3>x0
-----------------------
N0>U,N3,N2
N3>x1
N2>U,N3,N4
N3>x1
N4>x0
-----------------------
N0>|,N3,N2
N3>x1
N2>U,N3,N4
N3>x1
N4>x0
-----------------------
N0>|,N1,N2
N1>x0
N2>G,N3
N3>x1
-----------------------
N0>U,N4,N2
N4>x0
N2>U,N3,N4
N3>x1
N4>x0
-----------------------
N0>|,N4,N2
N4>x0
N2>U,N3,N4
N3>x1
N4>x0
-----------------------
N0>|,N3,N2
N3>x0
N2>U,N3,N4
N3>x0
N4>x1
-----------------------

Asked By: NBS

||

Source

Answer 1

String replace and strip are not in-place. They return a copy of the original string with the modification made.

with open('grammars.txt', 'r') as file3:
    # read a list of lines into data
    data = file3.readlines()

mylist = []
# Assuming that you have loaded data into a lines variable. 

  
for line in data:
    if 'n' in line:
        line=line.strip('n')
        print('hello')
    if '|' in line:
       line=line.replace('|','||')
       print('hello')
    if '&' in line:
      line=line.replace('&','&&')  
      print('hello')
    mylist.append(line)  
print(mylist)

Most if not all of Python’s string-mutating functions work the same way.

Answered By: Michael Sohnen

Answer 2

In addition to Michael Shonen’s terrific answer, it’s also worth noting that both strip and replace simply return the string unchanged if the specified characters do not appear in the original string.

Because of this those methods can be chained and it makes using conditional statements to check for their existence unnecessary.

With all that in mind, the script could be rewritten like this:

with open('grammars.txt', 'r') as file3:
    data = file3.readlines()

mylist = []

for line in data:
    line = line.strip('n').replace('|','||').replace('&','&&')
    mylist.append(line)

print(mylist)

or you could even do:

with open('grammars.txt', 'r') as file3:
    data = file3.readlines()
mylist = [line.strip('n').replace('|','||').replace('&','&&') for line in data]

Answered By: Alexander

replacing certain substrings in lines of a text file

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