Python file doesn’t run properly in PowerShell (Problem solved itself?)

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Summary

A Python script using colorama and termcolor to print colored text to the console appeared to “do nothing” when executed in PowerShell, while the same code executed successfully in the Python REPL. The core issue was the immediate termination of the console window (a common behavior in double-clicked .py files or certain launch contexts) combined with the lack of standard output buffering handling in terminal libraries. The problem resolved itself, likely due to the script being run from a persistent shell session rather than a fleeting window, ensuring the output remained visible long enough to be read.

Root Cause

The root cause was the execution context and the lifecycle of the console window. When a Python script is executed via a method that spawns a new, temporary console window (such as double-clicking the file or using specific GUI launchers), the Python process runs to completion, prints the colored output, and the process terminates. The operating system immediately closes the console window associated with that process, preventing the user from seeing the output.

Specific technical factors included:

  • Execution Method: Running python .\someScript.py from an existing PowerShell instance creates a child process that shares the console. Running the script directly (e.g., double-clicking) often spawns a new console window that is destroyed upon script completion.
  • Buffering: While colorama modifies sys.stdout to handle ANSI escape codes on Windows, it does not inherently change the stdout buffer mode to unbuffered. Python typically buffers stdout in line-buffered or full-buffered modes when not running interactively. If the script finishes and the window closes before the buffer is flushed, no visible output occurs.
  • Silent Failures: Python exceptions or syntax errors that occur before the print statement (e.g., failed imports) might be swallowed by the closing window if stderr is not captured, leaving the user with a blank screen.

Why This Happens in Real Systems

This behavior is a standard Windows console subsystem artifact and is frequently encountered in production environments where scripts are executed as scheduled tasks or standalone executables.

  • Console Lifecycle Management: In Windows, a process attached to a console is responsible for its lifetime. If the parent process (the launcher) terminates the child (the script) and closes the handle, the console disappears instantly.
  • Output Buffering in Headless Environments: In automated pipelines or CI/CD runners, if a script does not explicitly flush stdout (e.g., via sys.stdout.flush() or print(..., flush=True)), logs may be lost during abrupt process terminations or crashes.
  • Dependency Resolution: The silent failure suggests that the environment path might have been slightly different when running from the Mu editor vs. the REPL, potentially affecting the visibility of import errors.

Real-World Impact

While the symptom here was “no output,” the underlying mechanisms cause significant operational issues in production systems:

  • Observability Gaps: If a critical script fails silently because the window closes or the buffer isn’t flushed, operators have no logs to diagnose the failure, leading to silent data loss or unexecuted business logic.
  • Debugging Difficulty: Developers waste time investigating “ghost failures” where the code logic is correct but the execution environment prevents feedback.
  • Intermittent Failures: In automated task schedulers (like Windows Task Scheduler), scripts configured to run in “Hidden” windows may fail without any traceable error logs if output redirection isn’t configured correctly.

Example or Code

No code is required to reproduce the specific error, as the code provided in the input is syntactically correct and functional. The resolution depends on execution context, not logic. However, to prevent similar issues in production, one should ensure proper flushing.

import sys
from colorama import init
from termcolor import colored

# Initialize colorama
init()

# Print with explicit flushing to ensure output is visible
# even if the script is about to terminate immediately.
print(colored('WOWOWOWOWWO', 'green', 'on_red'))
sys.stdout.flush()

How Senior Engineers Fix It

Senior engineers address this by enforcing explicit execution contexts and robust output handling rather than relying on implicit environment behaviors.

  1. Standardize Execution Environments: Run scripts exclusively through a persistent shell (like PowerShell or Bash) rather than double-clicking files. For debugging, use the command line: python -u script.py (the -u flag forces unbuffered stdout/stderr).
  2. Implement Logging: Instead of relying on print(), use the logging module configured to write to both the console and a file. This decouples output from the console window lifecycle.
  3. Containerization: In complex production scenarios, wrap scripts in Docker containers or scheduled tasks that capture stdout/stderr to log files automatically.
  4. Input Validation: Add a input("Press Enter to exit...") at the end of scripts during development phases to keep the window open for inspection, though this is not a solution for production automation.

Why Juniors Miss It

Junior developers often lack awareness of the underlying operating system mechanics regarding process management and I/O buffering.

  • Expectation of Persistence: They assume that “running a script” is a monolithic action where the output stays on screen. They do not account for the fact that the window is a separate entity from the process and that the window closes when the process finishes.
  • Lack of Debugging Tools: They often do not know how to inspect environment variables or verify import paths (pip list or sys.path), leading them to miss subtle environment mismatches between the REPL and the script execution.
  • Reliance on GUIs: Using IDEs like Mu Editor or VS Code terminals abstracts away the console handling. When moving to a raw OS shell (PowerShell), the abstraction layer is removed, and the raw behavior of the Windows console subsystem becomes visible.