import and from Are Assignments

Just like def, import and from are executable statements, not compile-time declarations. They may be nested in if tests, appear in function defs, and so on, and they are not resolved or run until Python reaches them while executing your program. In other words, imported modules and names are not available until their associated import or from statements run. Also, like def, import and from are implicit assignments:

 

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  • import assigns an entire module object to a single name.
  • from assigns one or more names to objects of the same names in another module.

All the things we’ve already discussed about assignment apply to module access, too. For instance, names copied with a from become references to shared objects; as with function arguments, reassigning a fetched name has no effect on the module from which it was copied, but changing a fetched mutable object can change it in the module from which it was imported. To illustrate, consider the following file, small.py:

x = 1
y = [1, 2]

% python
>>> from small import x, y         # Copy two names out
>>> x = 42                         # Changes local x only
>>> y[0] = 42                      # Changes shared mutable in-place

Here, x is not a shared mutable object, but y is. The name y in the importer and the importee reference the same list object, so changing it from one place changes it in the other:

>>> import small                   # Get module name (from doesn't)
>>> small.x                        # Small's x is not my x
1
>>> small.y                        # But we share a changed mutable
[42, 2]

For a graphical picture of what from assignments do with references, flip back to Figure 18-1 (function argument passing), and mentally replace “caller” and “function” with “imported” and “importer.” The effect is the same, except that here we’re dealing with names in modules, not functions. Assignment works the same everywhere in Python.