How to Call a Python Script from COBOL

Integrating modern Python scripts with legacy COBOL applications can enhance functionality without rewriting entire systems. This guide explains the most efficient way to call a Python script from COBOL using widely used modules.

Why Integrate Python with COBOL?

COBOL remains a backbone for many enterprise systems, especially in banking and finance. Python, with its rich libraries, can extend COBOL applications with capabilities like data analysis, machine learning, or API integrations.

Prerequisites

A working COBOL environment (e.g., GnuCOBOL or IBM COBOL).
Python installed on the same system.
Basic familiarity with both languages.

Method: Using Python's `subprocess` Module

The most reliable way to call a Python script from COBOL is by using the subprocess module in Python. This method ensures cross-platform compatibility and efficient execution.

Step 1: Write the Python Script

Create a Python script (script.py) that performs the required task. For example:

# script.py
def process_data(input_data):
    return f"Processed: {input_data}"

if __name__ == "__main__":
    import sys
    input_data = sys.argv[1]  # Read input from COBOL
    result = process_data(input_data)
    print(result)  # Output to COBOL

Step 2: Call the Script from COBOL

In COBOL, use the CALL or SYSTEM command to execute the Python script. Here’s an example using GnuCOBOL:

IDENTIFICATION DIVISION.
PROGRAM-ID. CALL-PYTHON.
DATA DIVISION.
WORKING-STORAGE SECTION.
01 PYTHON-CMD PIC X(100) VALUE "python script.py 'Hello from COBOL'".
01 RESULT PIC X(100).

PROCEDURE DIVISION.
    CALL "SYSTEM" USING PYTHON-CMD
    DISPLAY "Python output: " RESULT.
    STOP RUN.

Step 3: Capture Output (Optional)

To capture the Python script's output in COBOL, redirect the output to a file and read it back:

IDENTIFICATION DIVISION.
PROGRAM-ID. READ-PYTHON-OUTPUT.
DATA DIVISION.
WORKING-STORAGE SECTION.
01 PYTHON-CMD PIC X(100) VALUE "python script.py 'Input' > output.txt".
01 FILE-CONTENTS PIC X(100).

PROCEDURE DIVISION.
    CALL "SYSTEM" USING PYTHON-CMD
    OPEN INPUT "output.txt"
    READ "output.txt" INTO FILE-CONTENTS
    DISPLAY "Python output: " FILE-CONTENTS
    CLOSE "output.txt"
    STOP RUN.

Alternative: Using a Shared File or Database

For more complex data exchanges, consider:

Writing output to a shared file (CSV/JSON).
Using a database (e.g., PostgreSQL) as an intermediary.

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