将新字符串添加到文本文件中特定行的末尾

Question

I'm new to python hence I am unable to implement the solutions I've found online in order to fix my problem. I am trying to add a specific string to the end of a specific line to a textfile. As I understand text commands, I must overwrite the file if I don't want to append to the end of it. So, my solution is as follows:

    ans = 'test'
numdef = ['H',2] 
f = open(textfile, 'r')
lines = f.readlines()
f.close()
f = open(textfile, 'w')
f.write('')
f.close()
f = open(textfile, 'a')
for line in lines:
    if int(line[0]) == numdef[1]:
        if str(line[2]) == numdef[0]:
                k = ans+ line
                f.write(k)
    else:
        f.write(line)

Basically, I am trying to add variable ans to the end of a specific line, the line which appears in my list numdef . So, for example, for

2 H: 4,0 : Where to search for information : google

I want

2 H: 4,0 : Where to search for information : google test

I have also tried using line.insert() but to no avail.

I understand using the 'a' function of the open command is not so relevant and helpful here, but I am out of ideas. Would love tips with this code, or if maybe I should scrap it and rethink the whole thing. Thank you for your time and advice!

Answer 1

Try this. You don't have an else case if it meets the first requirement but not the other.

ans = 'test'
numdef = ['H',2] 
f = open(textfile, 'r')
lines = f.readlines()
f.close()
f = open(textfile, 'w')
f.write('')
f.close()
f = open(textfile, 'a')
for line in lines:
    if int(line[0]) == numdef[1] and str(line[2]) == numdef[0]:
        k = line.replace('\n','')+ans
        f.write(k)
    else:
        f.write(line)
f.close()

Better way:

#initialize variables
ans = 'test' 
numdef = ['H',2]  
#open file in read mode, add lines into lines
with open(textfile, 'r') as f:
    lines=f.readlines() 
#open file in write mode, override everything    
with open(textfile, 'w') as f: 
    #in the list comprehension, loop through each line in lines, if both of the conditions are true, then take the line, remove all newlines, and add ans. Otherwise, remove all the newlines and don't add anything. Then combine the list into a string with newlines as separators ('\n'.join), and write this string to the file.
    f.write('\n'.join([line.replace('\n','')+ans if int(line[0]) == numdef[1] and str(line[2]) == numdef[0] else line.replace('\n','') for line in lines]))

Answer 2

When you use the method

lines = f.readlines()

Python automatically adds "\\n" to the end of each line.

Try instead of :

k = line+ans

The following:

k = line.rstrip('\n') + ans

Good luck!