import and from Equivalence

Notice in the prior example that we have to execute an import statement after the from to access the small module name at all. from only copies names from one module to another; it does not assign the module name itself. At least conceptually, a from statement like this one:

from module import name1, name2     # Copy these two names out (only)

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is equivalent to this statement sequence:

import module                       # Fetch the module object
name1 = module.name1                # Copy names out by assignment
name2 = module.name2
del module                          # Get rid of the module name

Like all assignments, the from statement creates new variables in the importer, which initially refer to objects of the same names in the imported file. Only the names are copied out, though, not the module itself. When we use the from * form of this statement (from module import *), the equivalence is the same, but all the top-level names in the module are copied over to the importing scope this way.

Notice that the first step of the from runs a normal import operation. Because of this, the from always imports the entire module into memory if it has not yet been imported, regardless of how many names it copies out of the file. There is no way to load just part of a module file (e.g., just one function), but because modules are byte code in Python instead of machine code, the performance implications are generally negligible.