Loading Modules and Packages

The process of importing a module/package (after locating it):

• First checks if it is cached. If not, continue
• It creates a ModuleType object with that name
• Cache the module in sys.modules
• Executes the source code inside the module (first prefixing it with .py and then assign __file__)
  • In the case of the package/subpackage, it assign it the __init__.py file
  • It also executes all the __init__.py on the path
• Assign a variable to the module object

Below is in roughly what it is done in Python code:

import sys, types

def import_module(modname):
    # Check if it is in the cache first
    if modname in sys.modules:
        return sys.modules[modname]

    sourcepath = modname + '.py'
    with open(sourcepath, 'r') as f:
        sourcecode = f.read()
    mod = types.ModuleType(modname)
    mod.__file__ = sourcepath

    # Cache the module
    sys.modules[modname] = mod

    # Convert it to Python ByteCode
    code = compile(sourcecode, sourcepath, 'exec')

    # Execute the code in the module from top to bottom
    # And update the state (globals) in the module's dictionary
    exec(code, mod.__dict__)

    # We return the cached one in case there is some patching inside the module
    return sys.modules[modname]

Finally, Python puts a lock when importing a module until it is done so that we don’t have multiple threads trying to import the same module at the same time.