How does Python receive stdin and arguments differently?

I'm not exactly sure what is confusing you here. stdin and command line arguments are treated as two different things.

Since you're most likely using CPython (the C implementation of Python) the command line args are passed automatically in the argv parameter as with any other c program. The main function for CPython (located in python.c) receives them:

int main(int argc, char **argv)  // **argv <-- Your command line args
{
    wchar_t **argv_copy;   
    /* We need a second copy, as Python might modify the first one. */
    wchar_t **argv_copy2;
    /* ..rest of main omitted.. */

While the contents of the pipe are stored in stdin which you can tap into via sys.stdin.

Using a sample test.py script:

import sys

print("Argv params:\n ", sys.argv)
if not sys.stdin.isatty():
    print("Stdin: \n", sys.stdin.readlines())

Running this with no piping performed yields:

(Python3)jim@jim: python test.py "hello world"
Argv params:
  ['test.py', 'hello world']

While, using echo "Stdin up in here" | python test.py "hello world", we'll get:

(Python3)jim@jim: echo "Stdin up in here" | python test.py "hello world"
Argv params:
 ['test.py', 'hello world']
Stdin: 
 ['Stdin up in here\n']

Not strictly related, but an interesting note:

Additionally, I remembered that you can execute content that is stored in stdin by using the - argument for Python:

(Python3)jimm@jim: echo "print('<stdin> input')" | python -
<stdin> input

Kewl!