A simple Text editor as a beginner

simpedit.py

#Text Editor
#A simple text editor in beta mode
#as a beginner
__author__ = 'Lexis'

def checknewline(tl):
    '''to check trailing newlines'''
    if (len(tl)==3) and (''.join(tl[:])==''):
        return True

def writeto(filename):
    text=[]
    fileheader = raw_input('File Header: ')
    filename.write(fileheader)
    run = True
    n=1
    while run == True:
        ui = raw_input(str(n)+': ')
        n+=1
        if ui != '':
            filename.write('\n'+ui)
        else:
            text.append(ui)
        ifspace3 = checknewline(text)
        if ifspace3 == True:
            filename.close()
            run = False

def main():
    filename = raw_input('Enter filename: ')
    MODE = 'a+'
    f = open(filename, MODE)
    writeto(f)
    
    
if __name__ == '__main__':
    main()