Can anyone give me a quick tutorial in stdin and stdout in Python 3?

Question

I know this sounds like something I can google, but the truth is that I don't find or do not understand what the very few Python 3 sources explains.

So here are my questions:

• Is input() the stdin function in Python 3? Does that mean that when you open your filename.py program, the stdin is what the user types?
• Is print() the stdout function in Python 3, or do you have to write to a file?
• For the Spotify puzzle, is says "Input is read from stdin". What should my file include of stdin and stdout?

---

Update: Does that mean that i can use:

import sys
unfmtdDate = str(sys.stdin.read())

...instead of...

unfmtdDate = str(input())

?

Solution 1

stdin and stdout are file-like objects provided by the OS. In general, when a program is run in an interactive session, stdin is keyboard input and stdout is the user's tty, but the shell can be used to redirect them from normal files or piped output from and input to other programs.

input() is used to prompt the user for typed input. In the case of something like a programming puzzle, it's normally assumed that stdin is redirected from a data file, and when the input format is given it's usually best to use sys.stdin.read() rather than prompting for input with input(). input() is intended for interactive user input, it can display a prompt (on sys.stdout) and use the GNU readline library (if present) to allow line editing, etc.

print() is, indeed, the most common way of writing to stdout. There's no need to do anything special to specify the output stream. print() writes to sys.stdout if no alternate file is given to it as a file= parameter.

Solution 2

When you run your Python program, sys.stdin is the file object connected to standard input (STDIN), sys.stdout is the file object for standard output (STDOUT), and sys.stderr is the file object for standard error (STDERR).

Anywhere in the documentation you see references to standard input, standard output, or standard error, it is referring to these file handles. You can access them directly (sys.stdout.write(...), sys.stdin.read() etc.) or use convenience functions that use these streams, like input() and print().

For the Spotify puzzle, the easiest way to read the input would be something like this:

import sys
data = sys.stdin.read()

After these two lines the input for your program is now in the str data.

Solution 3

Is input() the stdin function in Python 3?

Yes.

Does that mean that when you open your filename.py program, the stdin is what the user types?

Yes.

Is print() the stdout function in Python 3, or do you have to write to a file?

You can use either.

For the Spotify puzzle, is says "Input is read from stdin". What should my file include of stdin and stdout?

Just use import sys to get to the sys.stdin and sys.stdout files. You don't need the import if you use the input and print built-in functions instead.

Solution 4

Is input() the stdin function in Python 3? Does that mean that when you open your filename.py program, the stdin is what the user types?

input() reads from the standard input (called stdin for short in some contexts), yes. No language that I know of has a function named stdin in the standard library. When you run the program in the usual way, what the user types is supplied to standard input, yes. That's what "standard input" means. There is a separate program that is feeding what the user types to standard input (and doing a couple of other convenient things like interpreting the backspace key). Your script is not what creates the window that text is typed into.

sys.stdin (i.e., the value stdin defined in the sys module) is an object that represents the standard input. This is provided so that you can use it in contexts where you're expected to specify "a file", to cause input to be read from the user instead of a file.

Is print() the stdout function in Python 3, or do you have to write to a file?

print() writes (by default; you can supply a file keyword argument to change this) to the standard output (called stdout for short in some contexts), yes. All the above caveats apply: stdout isn't a function in any language I've ever heard of, and a completely separate program is actually causing the output of your script to be displayed on screen in a pretty 80x24 white-text-on-black-background (or however you have it configured) box.

sys.stdout is an object that represents the standard output, used similarly to sys.stdin. You can use it explicitly with the print function, but there's no point: it's the default.

For the Spotify puzzle, is says "Input is read from stdin". What should my file include of stdin and stdout?

The problem specification means "use the input() function to receive input".

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Comments

[Greg Hewgill]

stout? stein? Have another beer!

[Karl Knechtel]

Note that input() cannot read from a specified file (there are ways to cause the standard input to be a file, but again, that's generally external to your program). It's not meant to be exactly symmetric with the print() function. After all, it displays a prompt... wouldn't make sense to display a prompt while reading from a file, see?

[Mike DeSimone]

@Greg Hewgill: My browser seems to have grown itself an autocorrect future. It's being annoying.