Fixing the Fortran Code Execution Error when Run through Python

Learn how to troubleshoot and fix issues when executing `Fortran` code through `Python`, ensuring smooth reading and writing of files.
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This video is based on the question https://stackoverflow.com/q/67569880/ asked by the user 'Hypernova' ( https://stackoverflow.com/u/12567397/ ) and on the answer https://stackoverflow.com/a/67571801/ provided by the user 'jsbueno' ( https://stackoverflow.com/u/108205/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Troubleshooting Fortran Code Execution Through Python

If you are working with integrated programming environments where you have to pass data between Python and Fortran, you may occasionally run into issues that can be frustrating. One common scenario involves getting unexpected or erroneous outputs when running Fortran code through Python after file manipulation. Let’s break down a specific case and its solution to help you avoid these pitfalls.

Understanding the Problem

In your code, you're using Python to create a text file (fortc.txt) that will be read by a Fortran executable (f90code.exe). The expected workflow is as follows:

Python creates fortc.txt and writes some data into it.

Fortran reads from fortc.txt, processes the data, and outputs the results into res_tc.txt, which Python later reads and displays.

However, when running this process through Python, you encounter an error that makes Fortran read the contents of fortc.txt incorrectly, resulting in output that suggests the file is empty or contains invalid data. This can lead to floating-point exceptions and resulting errors in your output.

Identifying the Issue

The core problem here arises from the way Python handles file operations, specifically when using the with statement. The Python with statement automatically flushes and closes the file when exiting its block. Because the call to the Fortran executable occurs within this block, Python might not have completed writing the file before Fortran attempts to read it.

Key Observations:

Execution Order: Fortran seems to be trying to read fortc.txt before Python has finished writing to it.

Error Messages: The resulting error indicates that Fortran cannot properly process the file, reading everything as zero, which suggests an empty or uninitialized input.

The Solution

To resolve this issue, it’s essential to guarantee that the data has been fully written to fortc.txt before the Fortran executable is launched. Here’s the revised code workflow with proper restructuring:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Changes:

Separation of File Management: The code now handles the creation of fortc.txt and res_tc.txt separately, ensuring that fortc.txt is fully written and the file is closed before the Fortran code is executed.

Open and Close for res_tc.txt: The file res_tc.txt is specifically opened in the Python code without being nested inside the with block for fortc.txt.

Conclusion

By restructuring how files are handled in your Python script, you can ensure smooth communication between Python and Fortran. This small change helps mitigate issues related to file timing and ensures that the expected data is available for processing. Remember, when automating workflow between different programming languages, the sequence of operations can greatly affect your results.

If you run into similar issues, think about how files are managed and ensure that data is fully written before attempting to read it in a different context. Happy coding!