Non-Programmer'sTutorial for Python 3/File IO
File I/O
Hereis a simple example of file I/O (input/output):
# Write a file
with open("test.txt", "wt")
as out_file:
out_file.write("This Text is going to
out file\nLook at it and see!")
# Read a file
with open("test.txt", "rt")
as in_file:
text = in_file.read()
print(text)
Theoutput and the contents of the file test.txtare:
This Text is going to out file
Look at it and see!
Noticethat it wrote a file called test.txtin the directory that you ran the program from. The \nin the string tells Python to put a newlinewhere it is.
Anoverview of file I/O is:
-
Get a file object with the
openfunction -
Read or write to the file object (depending onhow it was opened)
-
Ifyou did not use
withto open the file, you'd have to close it manually
Thefirst step is to get a file object. The way to do this is to use theopenfunction. The format is file_object= open(filename, mode) wherefile_objectis the variable to put the file object, filenameis a string with the filename, and modeis "rt"to reada file as textor "wt"to writea file as text(and a few others we will skip here). Next the file objects functionscan be called. The two most common functions are readand write.The writefunction adds a string to the end of the file. The readfunction reads the next thing in the file and returns it as a string.If no argument is given it will return the whole file (as done in theexample).
Nowhere is a new version of the phone numbers program that we madeearlier:
def print_numbers(numbers):
print("Telephone Numbers:")
for k, v in numbers.items():
print("Name:", k,
"\tNumber:", v)
print()
def add_number(numbers, name, number):
numbers[name] = number
def lookup_number(numbers, name):
if name in numbers:
return "The number is " + numbers[name]
else:
return name + " was not found"
def remove_number(numbers, name):
if name in numbers:
del numbers[name]
else:
print(name," was not found")
def load_numbers(numbers, filename):
in_file = open(filename, "rt")
while True:
in_line = in_file.readline()
if not in_line:
break
in_line = in_line[:-1]
name, number = in_line.split(",")
numbers[name] = number
in_file.close()
def save_numbers(numbers, filename):
out_file = open(filename, "wt")
for k, v in numbers.items():
out_file.write(k + "," + v + "\n")
out_file.close()
def print_menu():
print('1. Print Phone Numbers')
print('2. Add a Phone Number')
print('3. Remove a Phone Number')
print('4. Lookup a Phone Number')
print('5. Load numbers')
print('6. Save numbers')
print('7. Quit')
print()
phone_list = {}
menu_choice = 0
print_menu()
while True:
menu_choice = int(input("Type in a
number (1-7): "))
if menu_choice == 1:
print_numbers(phone_list)
elif menu_choice == 2:
print("Add Name and Number")
name = input("Name: ")
phone = input("Number: ")
add_number(phone_list, name, phone)
elif menu_choice == 3:
print("Remove Name and Number")
name = input("Name: ")
remove_number(phone_list, name)
elif menu_choice == 4:
print("Lookup Number")
name = input("Name: ")
print(lookup_number(phone_list, name))
elif menu_choice == 5:
filename = input("Filename to
load: ")
load_numbers(phone_list, filename)
elif menu_choice == 6:
filename = input("Filename to
save: ")
save_numbers(phone_list, filename)
elif menu_choice == 7:
break
else:
print_menu()
print("Goodbye")
Noticethat it now includes saving and loading files. Here is some output ofmy running it twice:
1. Print Phone Numbers
2. Add a Phone Number
3. Remove a Phone Number
4. Lookup a Phone Number
5. Load numbers
6. Save numbers
7. Quit
Type in a number (1-7): 2
Add Name and Number
Name: Jill
Number: 1234
Type in a number (1-7): 2
Add Name and Number
Name: Fred
Number: 4321
Type in a number (1-7): 1
Telephone Numbers:
Name: Jill Number: 1234
Name: Fred Number: 4321
Type in a number (1-7): 6
Filename to save: numbers.txt
Type in a number (1-7): 7
Goodbye
1. Print Phone Numbers
2. Add a Phone Number
3. Remove a Phone Number
4. Lookup a Phone Number
5. Load numbers
6. Save numbers
7. Quit
Type in a number (1-7): 5
Filename to load: numbers.txt
Type in a number (1-7): 1
Telephone Numbers:
Name: Jill Number: 1234
Name: Fred Number: 4321
Type in a number (1-7): 7
Goodbye
Thenew portions of this program are:
def load_numbers(numbers, filename):
in_file = open(filename, "rt")
while True:
in_line = in_file.readline()
if not in_line:
break
in_line = in_line[:-1]
name, number = in_line.split(",")
numbers[name] = number
in_file.close()
def save_numbers(numbers, filename):
out_file = open(filename, "wt")
for k, v in numbers.items():
out_file.write(k + "," + v + "\n")
out_file.close()
Firstwe will look at the save portion of the program. First it creates afile object with the command open(filename,"wt"). Next it goesthrough and creates a line for each of the phone numbers with thecommand out_file.write(k+ "," + v + "\n").This writes out a line that contains the name, a comma, the numberand follows it by a newline.
Theloading portion is a little more complicated. It starts by getting afile object. Then it uses a whileTrue: loop to keep looping untila breakstatement is encountered. Next it gets a line with the line in_line= in_file.readline(). Thereadlinefunction will return an empty string when the end of the file isreached. The ifstatement checks for this and breaksout of the whileloop when that happens. Of course if the readlinefunction did not return the newline at the end of the line therewould be no way to tell if an empty string was an empty line or theend of the file so the newline is left in what readlinereturns. Hence we have to get rid of the newline. The line in_line= in_line[:-1] does this for usby dropping the last character. Next the line name,number = in_line.split(",")splits the line at the comma into a name and a number. This is thenadded to the numbersdictionary.
Advanced use of .txt files
Youmight be saying to yourself, "Well I know how to read and writeto a textfile, but what if I want to print the file without openingout another program?"
Thereare a few different ways to accomplish this. The easiest way doesopen another program, but everything is taken care of in the Pythoncode, and doesn't require the user to specify a file to be printed.This method involves invoking the subprocess of another program.
Rememberthe file we wrote output to in the above program? Let's use thatfile. Keep in mind, in order to prevent some errors, this programuses concepts from the Next chapter. Please feel free to revisit thisexample after the next chapter.
import subprocess
def main():
try:
print("This small program invokes
the print function in the Notepad application")
#Lets print the file we created in the program above
subprocess.call(['notepad','/p','numbers.txt'])
except WindowsError:
print("The called subprocess does
not exist, or cannot be called.")
main()
Thesubprocess.calltakes three arguments. The first argument in the context of thisexample, should be the name of the program which you would like toinvoke the printing subprocess from. The second argument should bethe specific subprocess within that program. For simplicity, justunderstand that in this program, '/p'is the subprocess used to access your printer through the specifiedapplication. The last argument should be the name of the file youwant to send to the printing subprocess. In this case, it is the samefile used earlier in this chapter.