Get all the information you could ever want about stdin, stdout, and stderr file descriptors in Python (e.g. is it a TTY, terminal, pipe, redirection, etc.)

test_stdin_stdout_stderr.py

#!/usr/bin/env python3
# Get all the information you could ever want about the STDIN, STDOUT, STDERR file descriptors (e.g. is it a TTY, terminal, pipe, redirection, etc.)
# Works cross-platform on Windows, macOS, Linux, in Docker, and in Docker-Compose by using stat library
#
# Useful for detecting and handling different stdin/stdout redirect scenarios in CLI scripts,
#     e.g. is the user piping a file in or are they interactively typing things in?
#          is the process output being saved to a file or being printed to a terminal?
#          can we ask the user for input or is it a non-interactive masquerading as a TTY?
#
# Further reading:
# - https://stackoverflow.com/questions/13442574/how-do-i-determine-if-sys-stdin-is-redirected-from-a-file-vs-piped-from-another
# - https://stackoverflow.com/questions/33871836/find-out-if-there-is-input-from-a-pipe-or-not-in-python
# - https://stackoverflow.com/questions/30137135/confused-about-docker-t-option-to-allocate-a-pseudo-tty

import os
import sys
import stat


def log_dict_summary(obj: dict):
    sys.stderr.write(' '.join(f'{key}={str(val).ljust(6)}' for key, val in obj.items()) + '\n')

def get_fd_info(fd):
    FILENO = fd.fileno()
    MODE = os.fstat(FILENO).st_mode
    IS_PIPE, IS_TERMINAL = stat.S_ISFIFO(MODE), stat.S_ISREG(MODE)
    return {
        'NAME': fd.name[1:-1],
        'FILENO': FILENO,
        'MODE': MODE,
        'IS_PIPE': IS_PIPE,
        'IS_FILE': IS_TERMINAL,
        'IS_TERMINAL': not (IS_PIPE or IS_FILE),
        'IS_TTY': hasattr(fd, 'isatty') and fd.isatty(),
        'IS_LINE_BUFFERED': fd.line_buffering,
        'IS_READABLE': fd.readable(),
    }
    

sys.stdout.write('[>&1] this is python stdout\n')
sys.stderr.write('[>&2] this is python stderr\n')
log_dict_summary(get_fd_info(sys.stdin))
log_dict_summary(get_fd_info(sys.stdout))
log_dict_summary(get_fd_info(sys.stderr))

### BASH STDIN/STDOUT/STDERR BEHAVIOR (as tested on macOS in bash)

    ### BASH w/ stdin

        # python3 test_stdin_stdout_stderr.py
        #   NAME=stdin  FILENO=0   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=True   IS_READABLE=True
        #   NAME=stdout FILENO=1   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=True   IS_READABLE=False
        #   NAME=stderr FILENO=2   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=True   IS_READABLE=False

        # echo test | python3 test_stdin_stdout_stderr.py
        #   NAME=stdin  FILENO=0   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=True
        #   NAME=stdout FILENO=1   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=True   IS_READABLE=False
        #   NAME=stderr FILENO=2   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=True   IS_READABLE=False


        # python3 test_stdin_stdout_stderr.py < testin
        #   NAME=stdin  FILENO=0   IS_TTY=False  IS_PIPE=False  IS_FILE=True   IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=True
        #   NAME=stdout FILENO=1   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=True   IS_READABLE=False
        #   NAME=stderr FILENO=2   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=True   IS_READABLE=False


    ### BASH w/ stdout
    
        # python3 test_stdin_stdout_stderr.py > testout
        #   NAME=stdin  FILENO=0   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=True   IS_READABLE=True
        #   NAME=stdout FILENO=1   IS_TTY=False  IS_PIPE=False  IS_FILE=True   IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=False
        #   NAME=stderr FILENO=2   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=True   IS_READABLE=False

        # python3 test_stdin_stdout_stderr.py | cat
        #   NAME=stdin  FILENO=0   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=True   IS_READABLE=True
        #   NAME=stdout FILENO=1   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=False
        #   NAME=stderr FILENO=2   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=True   IS_READABLE=False



#### DOCKER-COMPOSE STDIN/STDOUT/STDERR BEHAVIOR (as tested on macOS in bash)

    ### TTY w/ stdin

        # docker-compose run test                     (X docker sends stderr to terminal stdout)
        #   NAME=stdin  FILENO=0   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=False  IS_READABLE=True
        #   NAME=stdout FILENO=1   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=False  IS_READABLE=False
        #   NAME=stderr FILENO=2   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=False  IS_READABLE=False

        # docker-compose run test < testin            (√ docker sends stderr to terminal stderr!)
        #   NAME=stdin  FILENO=0   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=True
        #   NAME=stdout FILENO=1   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=False
        #   NAME=stderr FILENO=2   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=False

        # echo test | docker-compose run test         (√ docker sends stderr to terminal stderr!)
        #   NAME=stdin  FILENO=0   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=True
        #   NAME=stdout FILENO=1   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=False
        #   NAME=stderr FILENO=2   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=False

    ### TTY w/ stdout

        # docker-compose run test > testout          (X docker sends stderr to file as well)
        #   NAME=stdin  FILENO=0   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=False  IS_READABLE=True  
        #   NAME=stdout FILENO=1   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=False  IS_READABLE=False 
        #   NAME=stderr FILENO=2   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=False  IS_READABLE=False 

        # docker-compose run test | cat              (X docker sends stderr to pipe as well)
        #   NAME=stdin  FILENO=0   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=False  IS_READABLE=True  
        #   NAME=stdout FILENO=1   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=False  IS_READABLE=False 
        #   NAME=stderr FILENO=2   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=False  IS_READABLE=False 


    ### NON-TTY w/ stdin

        # docker-compose run -T test                 (√ docker sends stderr to terminal stderr)
        #   NAME=stdin  FILENO=0   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=True
        #   NAME=stdout FILENO=1   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=False
        #   NAME=stderr FILENO=2   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=False

        # docker-compose run -T test < testin        (√ docker sends stderr to terminal stderr)
        #   NAME=stdin  FILENO=0   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=True
        #   NAME=stdout FILENO=1   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=False
        #   NAME=stderr FILENO=2   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=False

        # echo test | docker-compose run -T test     (√ docker sends stderr to terminal stderr)
        #   NAME=stdin  FILENO=0   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=True
        #   NAME=stdout FILENO=1   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=False
        #   NAME=stderr FILENO=2   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=False

    ### NON-TTY w/ stdout

        # docker-compose run -T test > testout       (√ docker sends stderr to terminal stderr)
        #   NAME=stdin  FILENO=0   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=True
        #   NAME=stdout FILENO=1   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=False
        #   NAME=stderr FILENO=2   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=False



#### DOCKER STDIN/STDOUT/STDERR BEHAVIOR (tested on macOS in bash)

    ### TTY w/ stdin

        # docker run -it test                     (X docker sends stderr to terminal stdout)
        #   NAME=stdin  FILENO=0   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=False  IS_READABLE=True
        #   NAME=stdout FILENO=1   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=False  IS_READABLE=False
        #   NAME=stderr FILENO=2   IS_TTY=True   IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=False  IS_READABLE=False

        # docker run -it test < testin            (X stdin with TTY not allowed)

        # echo test | docker run -it test         (X stdin with TTY not allowed)

    ### TTY w/ stdout

        # docker run -it test > testout           (X docker sends stderr to file as well)
        #   
        #   (same behavior as docker-compose...)
        #   

        # docker run -it test | cat               (X docker sends stderr to pipe as well)
        #   
        #   (same behavior as docker-compose...)
        #   

    ### NON-TTY w/ no stdin
    
        # docker run test                        (X stdin is ignored, stderr goes to stderr)
        #   NAME=stdin  FILENO=0   IS_TTY=False  IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=False  IS_READABLE=True
        #   NAME=stdout FILENO=1   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=False
        #   NAME=stderr FILENO=2   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=False
      
        # docker run test < testin               (X stdin is ignored)
        #
        #
        #
        
        # echo test | docker run test            (X stdin is ignored)
        #   
        #   
        #   
        
      
    ### NON-TTY w/ stdin

        # docker run test                        (√ stdin->stdin, stdout->stdout, stderr->stderr)
        #   NAME=stdin  FILENO=0   IS_TTY=False  IS_PIPE=False  IS_FILE=False  IS_TERMINAL=True   IS_LINE_BUFFERED=False  IS_READABLE=True
        #   NAME=stdout FILENO=1   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=False
        #   NAME=stderr FILENO=2   IS_TTY=False  IS_PIPE=True   IS_FILE=False  IS_TERMINAL=False  IS_LINE_BUFFERED=False  IS_READABLE=False

        # docker run -i test < testin            (√ file->stdin, stdout->stdout, stderr->stderr)
        #   
        #   (same behavior as docker-compose...)
        #   

        # echo test | docker run -i test         (√ pipe->stdin, stdout->stdout, stderr->stderr)
        #   
        #   (same behavior as docker-compose...)
        #   

    ### NON-TTY w/ stdout

        # docker run test > testout              (√ null->stdin, stdout->file, stderr->stderr)
        #   
        #   (same behavior as docker-compose...)
        #   

# TODO: test behavior over SSH with -T and without