Python - read and write input from user

Question

I am having trouble writing this short program for my python class I was hoping someone could offer some assistance.

What I would like to accomplish: 1. Write a program that uses a while loop to accept input from the user (if the user presses Enter, exit the program). 2. Save the input to a file, then print it. 3. Upon starting, the program will display the current contents of the file.

Example:

Start program for first time.

Enter text: this is input

this is input.

Enter text: some more text

this is input. some more text.

When you start the program for a second time

this is input. some more text.

Enter text:

etc. etc.

What I have so far:

intext = open('user_input.txt','a')
intext.close()
string_input = input('Enter text: ')
while True:
    open_input = open('user_input.txt','r')
    if open_input:
        for i in open_input:
            print(i)

    if string_input != "":
        uinput = open('user_input.txt','a')
        uinput.write(string_input + '.')
        uinput.close()
        rd = open('user_input.txt', 'r')
        if rd:
            for line in rd:
                print(line)

    if string_input == "":
        t = open('user_input.txt', 'r')
        for line in t:
            print(line)
        t.close()
        break

Problems: Upon opening, any previously stored text does not display. If a user inputs text it prints in an infinite loop, and does not prompt to enter text again.

Positives: The input is recorded to the text file. If no text is entered, when exiting any previously entered text does display correctly.

Like I said, this is homework for me. I have searched for the answer, but I seem to be ripping the code apart and putting it back together only to get different errors. So some guidance on this would be greatly appreciated.

One thing I forgot to mention is that I am using Python 3.

Thanks again to David for helping me think more like a programmer. Here are the results:

intext = open('user_input.txt','r').readline()
print(intext)
while True:
    string_input = input('Enter text: ')
    if string_input == "":
        t = open('user_input.txt', 'r').readline()
        print(t)
        break
    if string_input != "":
        d = open('user_input.txt', 'a')
        d.write(string_input + '. ')
        d.close()
        n = open('user_input.txt', 'r').readline()
        print(n)

I tried to keep the code as slim as possible, and it works now.

A couple questions additional questions that came out of this:

Do I need to close the file at the end? When I tried to close apnd and n , It gave me errors.
While looking for answers I came a across, this. Is it still best practice to use a "with" statement?

Example:

with open("x.txt") as f:
    data = f.read()
    do something with data

David Z · Accepted Answer · 2011-11-23 23:55:41Z

To be honest, your program as you've shown it is kind of a mess. I say this not to be insulting, but because you do have a pretty clear list of the steps your program needs to take, and I think you will wind up with a better understanding by scrapping your existing code and starting from scratch.

In your question, you listed the following steps:

Upon starting, display any previous content of the file
Use a while loop to
1. Accept input from the user
2. If the user presses Enter, exit the program
3. Save the input to a file
4. Print it (note: did you mean to print just the latest input, or everything in the file?)

Turning your overall task into a list of specific steps like this is probably 80% of the work of writing a computer program. All that's left to do is translate it into code. So I would suggest that you consider how to do each of these steps individually.

Write a code snippet to display the contents of a file
Write a code snippet to read a line of input from the user and store it in a variable
Write a code snippet to check the contents of a variable to see whether it's empty, and if so, exit
Write a code snippet to append the contents of a variable to a file
Write a code snippet to print the contents of a variable (or of a file, if that's what you meant to do)

Each of these can be done in one or two lines, so individually, you should have an easy time with them. Once you've done all the pieces, all you need to do is put them together:

# display the contents of the file
while True:
    # read a line of input and store it in a variable
    # check the contents of the variable to see if it's empty, and if so, exit
    # append the contents of the variable to the file
    # print the contents of the variable (or of the file)

Update: This is not a big deal, but you have an unnecessary if statement (the second one) in your revised program. Think about this: if string_input is empty, Python will execute the break statement, which terminates the loop immediately. So you'll only ever reach the second if statement if string_input is not empty. That means the condition string_input != "" is guaranteed to be true at that point in the program, and there's no need to check it.

Do I need to close the file at the end? When I tried to close apnd and n , It gave me errors.

Yes you do. Look at the pattern you used with d:

# open the file
d = open('user_input.txt', 'a')
# write to it (or read from it)
d.write(string_input + '. ')
# close it
d.close()

You should do the same thing every time you open a file, namely with intext, t, and n: open it, read from it, and then immediately close it.^*

I'm guessing that the reason you encountered errors is that you tried to put the .close() statements at the end of the program, outside of the if statement and perhaps even outside of the while loop. That would give you a NameError because the variables n and t are not defined at those points in the program. They "expire" at the end of the block they are defined in. For more information on this, read up on scoping. (The Wikipedia article is probably not the best introduction, but you can search Stack Overflow and/or the web for more resources.)

While looking for answers I came a across, this. Is it still best practice to use a "with" statement?

Yes, the with statement is relatively new to Python and is the recommended way to do "quick" file I/O operations like this. Basically the with block takes care of closing the file at the end. For example, the code snippet above involving d is equivalent to

# open the file
with open('user_input.txt', 'a') as d:
    # write to it (or read from it)
    d.write(string_input + '. ')
# Python "automatically" closes it for you

^*This "open-read/write-close" pattern of file access is usually a good idea. I've told you to use it in your program because it's important for you to learn how to split a program into small steps and convert each of the steps into code individually. But when you are writing a program that repeatedly writes things out to a file, or reads them in from a file, sometimes it is actually better to just open the file once in the beginning and just keep it open, rather than opening and closing it every time. If you are curious, one thing you could investigate is how to modify your program to reduce the number of times it has to open and close the file.

thanks for the honesty. I will take a step back go with a more organized approach.

rechardchen · Accepted Answer · 2011-11-23 06:09:44Z

0

Use raw_input instead of input.

You forget to call input in the while loop

BTW, why not write data just at exit of the program instead of in each loop?

answered Nov 23, 2011 at 6:09

rechardchen

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