import os
file1 = '12.txt'
for root, dirs, files in os.walk(r'C:\Temp'):
    for name in files:
        if name == file1:
            print (os.path.abspath(os.path.join(root, name)))

 import os
for root, dirs, files in os.walk(r'C:\\'): print(files)

 for root, dirs, files in os.walk(r'C:\\'):
    #print(files)
    file1 = 'setup.log'
    for name in files:
        if name == file1:
            print (os.path.abspath(os.path.join(root, name)))
            break

 file1 = '\install\setup.log'

 import os
os.startfile('..\\..\\test.txt')

 import os, sys, msvcrt
def terminate():
    if msvcrt.kbhit(): 
        #print('process terminated')
        sys.exit()
file1 = 'test.txt'
print('Start search')
for root, dirs, files in os.walk(r'C:\\'):
    #print(files)
    for name in files:
        print(name)
        terminate()
        if name == file1:
            print (os.path.abspath(os.path.join(root, name)))
            break
print('Finish search')

 import os
namefile = '12.txt'
for root, dirs, files in os.walk(r'C:\\'):
    for name in files:
        if name == namefile:
            pathtofile = (os.path.abspath(os.path.join(root, name)))
            # print(pathtofile)
f = open(pathtofile,'r', encoding='cp1251')
filedata = f.read()
f.close()
newdata = filedata.replace("вася","вася\nмаша - козел")
f = open(pathtofile,'w')
f.write(newdata)
f.close()