[SOLVED] Python can't read script from stdin when input() appears in the script.

mimorek · 04-23-2021, 08:49 AM

Hello,

When I let python run a short script from stdin it works, but not when the input() function appears in the script.
What is behind the EOFError and is there a way to make this work?

Thanks.

This fails:

Code:

$ python3 <<HERE
> x = input()
> x = int(x)
> print(x**3)
> HERE
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
EOFError: EOF when reading a line

This works:

Code:

$ python3 <<HERE
> x = 3**3
> print(x)
> HERE
27

NevemTeve · 04-23-2021, 08:59 AM

When you redirect the stdin of python executable, it is not a Python-script. Python script is a file like this:

Code:

#!/usr/bin/python3
x = input()
x = int(x)
print(x**3)

Two ways to run it (assuming its name is `test3.py`):

Code:

python3 test3.py
chmod +x test3.py; ./test3.py

mimorek · 04-23-2021, 09:04 AM

But that still doesn't work.

Code:

$ python3 <<HERE
> #!/usr/bin/python3
> x = input()
> x = int(x)
> print(x**3)
> HERE
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
EOFError: EOF when reading a line

NevemTeve · 04-23-2021, 09:13 AM

Indeed, it doesn't work, that's why I suggest creating an actual script instead.

Edit: as an experiment, you could try this:

Code:

python3 <<HERE
x = input()
42
x = int(x)
print(x**3)
HERE

mimorek · 04-23-2021, 09:27 AM

I think I haven't explained my question well.
I am trying to run this from the Bash cli. I am not creating a file.

boughtonp · 04-23-2021, 09:32 AM

There's no need to create a file - as per "python3 --help" you can pass a string using -c option:

Code:

$ python3 -c 'print(int(input())**3)'
5
125

But I think mimorek's question is why the first of these works but the second doesn't:

Code:

$ echo 'print(int('5')**3)' | python3
125

$ echo 'print(int(input())**3)' | python3
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
EOFError: EOF when reading a line

I suspect the answer is something to do with because the script is passed via stdin, there is then no stdin for the script to read from.

When the script is not provided over stdin, then input() can be used to read from it:

Code:

$ echo 5 | python3 -c 'print(int(input())**3)'
125

Ser Olmy · 04-23-2021, 09:54 AM

Quote:

Originally Posted by mimorek

But that still doesn't work.

Code:

$ python3 <<HERE

So you're telling the shell to feed the python3 executable some text via stdin. OK.

Quote:

Originally Posted by mimorek

Code:

> x = input()

And then you wonder why stdin isn't available for reading input?

michaelk · 04-23-2021, 09:56 AM

boughtonp is correct python no longer has access to stdin and there are a few ways to work around the problem. Here is your code as posted.

Code:

#!/bin/bash
python3 <(cat <<HERE
x = input()
x = int(x)
print(x**3)
HERE
)

#!/bin/bash
python3 -c '
x = input()
x = int(x)
print(x**3)
'

pan64 · 04-23-2021, 10:07 AM

Code:

$ python3 <<HERE
> x = 3**3
> print(x)
> HERE
27

here python3 will read stdin and take it as a script (and execute it).

Code:

$ python3 <<HERE
> #!/usr/bin/python3
> x = input()
> x = int(x)
> print(x**3)
> HERE

here python3 will read stdin and x=input will also read the same stdin. The result is that python3 will not find the end of the file (because slurped by that input call).

NevemTeve · 04-23-2021, 11:36 PM

I tried the suggested experiment, it doesn't work either. The reason is simple: python3 executable reads the input in blocks (4096 byte in my example).
Part of `strace` output:

Code:

read(0, "x = input()\n42\nx = int(x)\nprint("..., 4096) = 38
read(0, "", 4096)                       = 0
...
read(0, "", 8192)                       = 0

The first `read` gets everything (38 bytes); the second `read` returns 0 to indicate end-of-file (EOF), the third `read` is the `input` in the program which also returns 0 (end-of-file).

mimorek · 04-24-2021, 08:14 AM

Thanks for responding everybody.
It makes more sense to me now.

The answer michaelk provided kept me puzzled and searching for quite a while:

Code:

python3 <(cat <<HERE
x = input()
x = int(x)
print(x**3)
HERE
)

But now I've learned that process substitution treats its output as a file and it is no longer stdin:

Code:

python3 <( cat <<HERE
import sys
print(sys.argv)
HERE
)
['/dev/fd/63']      <-- output

from the bash manpage:

"Process Substitution
Process substitution allows a process's input or output to be referred to using a filename."