Replace string within file contents

with open("out.txt", "wt") as out:
    for line in open("Stud.txt"):
        out.write(line.replace('A', 'Orange'))

with open('Stud.txt','r') as f:
    newlines = []
    for line in f.readlines():
        newlines.append(line.replace('A', 'Orange'))
with open('Stud.txt', 'w') as f:
    for line in newlines:
        f.write(line)

text_file = open("file.txt", "rw")
whole_thing = text_file.read()
whole_thing = whole_thing.replace("A", "Orange");
text_file.write(whole_thing);
text_file.close();

file = open('Stud.txt')
contents = file.read()
replaced_contents = contents.replace('A', 'Orange')

<do stuff with the result>

import re

input = file('C:full_pathStud.txt), 'r')
#when you try and write to a file with write permissions, it clears the file and writes only #what you tell it to the file.  So we have to save the file first.

saved_input
for eachLine in input:
    saved_input.append(eachLine)

#now we change entries with 'A' to 'Orange'
for i in range(0, len(old):
    search = re.sub('A', 'Orange', saved_input[i])
    if search is not None:
        saved_input[i] = search
#now we open the file in write mode (clearing it) and writing saved_input back to it
input = file('C:full_pathStud.txt), 'w')
for each in saved_input:
    input.write(each)