Prevent Python packages from re-exporting imported names

There is no easy way to forbid importing a global name from a module; Python simply is not built that way.

While you could possibly achieve the forbidding goal if you wrote your own __import__ function and shadowed the built-in one, but I doubt the cost in time and testing would be worth it nor completely effective.

What you can do is import the dependent modules with a leading underscore, which is a standard Python idiom for communicating "implementation detail, use at your own risk":

import re as _re
import sys as _sys

def hello():
    pass

Note

While just deleting the imported modules as a way of not allowing them to be imported seems like it might work, it actually does not:

import re
import sys

def hello():
    sys
    print('hello')

del re
del sys

and then importing and using hello:

>>> import del_mod
>>> del_mod.hello()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "del_mod.py", line 5, in hello
    sys
NameError: global name 'sys' is not defined

Update. Some experience later, I'd strongly encourage the use of __all__, and discourage the initializer-function idea. There is a lot of tooling, that will be confused by it.

1. Initializer function

An alternative might be wrapping definitions into an initializer function.

## --- exporttest.py ---
def _init():
    import os                       # effectively hidden

    global get_ext                  # effectively exports it
    def get_ext(filename):
        return _pointless_subfunc(filename)

                                      # underscore optional, but good 
    def _pointless_subfunc(filename): # for the sake of documentation
        return os.path.splitext(filename)[1]

    if __name__ == '__main__':      # for interactive debugging (or doctest)  
        globals().update(locals())  # we want all definitions accessible
        import doctest
        doctest.testmod()
        
_init()

print('is ``get_ext`` accessible?           ', 'get_ext' in globals())
print('is ``_pointless_subfunc`` accessible?', '_pointless_subfunc' in globals())
print('is ``os`` accessible?                ', 'os' in globals())

For comparison:

>>> python3 -m exporttest
is ``get_ext`` accessible?            True
is ``_pointless_subfunc`` accessible? True
is ``os`` accessible?                 True

>>> python3 -c "import exporttest"
is ``get_ext`` accessible?            True
is ``_pointless_subfunc`` accessible? False
is ``os`` accessible?                 False

1.1. Advantages

  • Actual hiding of the imports.
  • More convenient for interactive code-exploration, as dir(exporttest) is clutter-free.

1.2. Disadvantages

  • Sadly, unlike the import MODULE as _MODULE pattern, it doesn't play nicely with pylint.

    C:  4, 4: Invalid constant name "get_ext" (invalid-name)
W:  4, 4: Using global for 'get_ext' but no assignment is done (global-variable-not-assigned)
W:  5, 4: Unused variable 'get_ext' (unused-variable)

  • It also doesn't play nicely with IDE intellisense features.

2. Embrace __all__

Upon further reading, I've found that the pythonic way to do it is to rely on __all__. It controls not only what is exported on from MODULE import *, but also what shows up in help(MODULE), and according to the "We are all adults here" mantra, it is the users own fault if he uses anything not documented as public.

2.1. Advantages

Tooling has best support for this approach (e.g. through editor support for autoimports through the importmagic library).

2.2. Disadvantages

Personally, I find that whole "we are all adults" mantra quite naive; When working under time pressure with no chance to fully understand a code-base before delivering a change, we can do with any help we can get to prevent "shot your own foot" scenarios. Plus, even many popular packages don't really follow best practices like providing useful interactive docstrings, or defining __all__. But it is the pythonic way.

Tags:

Python

Module

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