python是否有类似C ++的using关键字的东西？

Question

In C++ you can often drastically improve the readability of your code by careful usage of the "using" keyword, for example:

void foo()
{
    std::vector< std::map <int, std::string> > crazyVector;
    std::cout << crazyVector[0].begin()->first;
}

becomes

void foo()
{
    using namespace std; // limited in scope to foo
    vector< map <int, string> > crazyVector;
    cout << crazyVector[0].begin()->first;
}

Does something similar exist for python, or do I have to fully qualify everything?

I'll add the disclaimer that I know that using has its pitfalls and it should be appropriately limited in scope.

Answer 1

As Bill said, Python does have the construction

from X import *

but you can also explicitly specify which names you want imported from the module (namespace):

from X import foo, bar, blah

This tends to make the code even more readable/easier to understand, since someone seeing an identifier in the source doesn't need to hunt through all imported modules to see where it comes from. Here's a related question: Namespace Specification In Absence of Ambuguity

EDIT : in response to Pax's comment, I'll mention that you can also write things like

import X.foo

but then you'll need to write

X.foo.moo()

instead of just

foo.moo()

This is not necessarily a bad thing, of course. I usually use a mixture of the from X import y and import Xy forms, whatever I feel makes my code clearest. It's certainly a subjective thing to some extent.

Answer 2

import X

or

from X import *

or

from X import a, b, c

Where X is the Python module you want to use.

It would be helpful for you to give us a Python code sample that you think needs cleaned up.

Answer 3

Sure, python's dynamism makes this trivial. If you had a class buried deep in a namespace: foo.bar.baz.blah, you can do:

def foo:
    f = foo.bar.baz.blah
    f1 = f()

Answer 4

Another option in Python is the construct "as" when importing. For example:

from foo import bar as baz

This will bring foo.bar in as baz in the current module which allows the entity to get a different name in the current module, possibly to avoid hiding an existing entity in the current module with the same name.

Answer 5

In addition to David's answer:

1. One should use round brackets in from X import (foo, bar, blah) for a sake of PEP8 .
2. Full syntax allows renaming (rebinding) package names to their new identifiers within the scope of the current module as in from foo import bar as baz .

I recommend to look up the manual for the import keyword , the __import__ built-in and explanation for sys.modules as further reading.

Answer 6

Note that

from foo import bar

works even if bar is a module in the foo package. This lets you limit your namespace pollution without having to name each function/class in foo.bar that you might care to use. It also aids readers of your code, because they'll see a call to bar.baz() and have a better idea where baz came from.