Saturday, November 22, 2014

Running Python from Python

It has been pointed out to me that I don't blog as often as I used to. So here's a first step towards rectifying that.
In this post, I'm going to briefly describe the support that Python provides for processing, well, "Python". If you're using Python for simple scripting and automation tasks, you might often have to load, parse and execute other Python files from your code. While you can always "import" some Python code as a module, and execute it, in many situations it is impossible to determine precisely at the development time, which Python files your code needs to import. Also some Python scripts are written as simple executable files, which are not ideal for inclusion via import. To deal with cases such as these, Python provides several built-in features that allow referring to and executing other Python files.
One of the easiest ways to execute an external Python file is by using the built-in execfile function. This function takes the path to another Python file as the only mandatory argument. Optionally, we can also provide a global and a local namespace. If provided, the external code will be executed within those namespace contexts. This is a great way to exert some control over how certain names mentioned in the external code will be resolved (more on this later).
Another way to include some external code in your script is by using the built-in __import__ function. This is the same function that gets called when we use the usual "import" keyword to include some module. But unlike the keyword, the __import__ function gives you lot more control over certain matters like namespaces.
Another way to run some external Python code from your Python script is to first read the external file contents into memory (as a string), and then use the exec keyword on it. The exec keyword can be used as a function call or as keyword statement.
code_string = load_file_content('/path/to/')
Similar to the execfile function, you have the option of passing custom global and local namespaces. Here's some code I've written for a project that uses the exec keyword:
globals_map = globals().copy()
globals_map['app'] = app
globals_map['assert_app_dependency'] = assert_app_dependency
globals_map['assert_not_app_dependency'] = assert_not_app_dependency
globals_map['assert_app_dependency_in_range'] = assert_app_dependency_in_range
globals_map['assert_true'] = assert_true
globals_map['assert_false'] = assert_false
globals_map['compare_versions'] = compare_versions
    exec(self.source_code, globals_map, {})
except Exception as ex:
    utils.log('[{0}] Unexpected policy exception: {1}'.format(, ex))
Here I first create a clone of the current global namespace, and pass it as an argument to the exec function. The clone is discarded at the end of the execution. This makes sure that the code in the external file does not pollute my existing global namespace. I also add some of my own variables and functions (e.g assert_true, assert_false etc.) into the global namespace clone, which allows the external code to refer to them as built-in constructs. In other words, the external script can be written in a slightly extended version of Python.
There are other neat little tricks you can do using the constructs like exec and execfile. Go through the official documentation for more details.

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