How to replace/change element in txt.file

Question

Im trying to replace a cetain element in a txt file. let say that if i find the name in telephonelist.txt, i want i to change the number to this person with the input value of newNumber. let's say that name = Jens, then i want it to return 99776612 that is the tlf number to Jens, and then the input of 'newNumber' will replace this number. i am new to python.

def change_number():
while True:
    try:
      name = input('Name: ') #Enter name
      newNumber = input('New number: ') # Wanted new number
      datafile = open('telephonelist.txt')

      if name in open('telephonelist.txt').read():
        for line in datafile:
          if line.strip().startswith(name):
            line = line.replace(name,newNumber)
            print('I found', name)
            quit()
      else:
        print('I could not find',name+',','please try again!\n')
        continue
    except ValueError:
      print('nn')
change_number()



This i telephonelist.txt

    Kari 98654321
    Liv  99776655
    Ola  99112233
    Anne 98554455
    Jens 99776612
    Per  97888776
    Else 99455443
    Jon  98122134
    Dag  99655732
    Siv  98787896

Answer 1

Load content, modify it, seek to beginning of the file, write the modified content again and then truncate the rest.

def change_number():
    name = input('Name: ') #Enter name
    newNumber = input('New number: ') # Wanted new number
    with open('telephonelist.txt', 'r+') as file:
        data = file.read().splitlines()
        data = [line if not line.split()[0] == name else f"{name} {newNumber}" for line in data]
        file.seek(0)
        file.write("\n".join(data))
        file.truncate()

change_number()