Why isn't this Python import working?

I have three modules:

plugin_grt.py

fragments.py

helpers.py

Right at the top of plugin_grt.py I have

from jpa_export_helpers import SourceFile, Mysql, Conv, Columns, Column, Table, ForeignKey, Index, Catalog, Inheritance

which works, that is I can use Table.whateverMethod(...) without problems. Now when I add the same import to the top of the fragments.py module I get:

Traceback (most recent call last):

  File "C:\Users\Kawu\AppData\Roaming\MySQL\Workbench\modules\jpa_export_plugin_grt.py", line 53, in <module>
    from jpa_export_helpers import SourceFile, Mysql, Conv, Columns, Column, Table, ForeignKey, Index, Catalog, Inheritance

  File "C:\Users\Kawu\AppData\Roaming\MySQL\Workbench\modules\jpa_export_helpers.py", line 2, in <modul开发者_开发技巧e>
    from jpa_export_fragments import Type, EnumValue

  File "C:\Users\Kawu\AppData\Roaming\MySQL\Workbench\modules\jpa_export_fragments.py", line 2, in <module>
    from jpa_export_helpers import SourceFile, Mysql, Conv, Columns, Column, Table, ForeignKey, Index, Catalog, Inheritance

ImportError: cannot import name SourceFile

Why isn't this working? The only workaround is to import the classes right where they're needed, but it's not something I like (at least for now):

def getPrimaryKeyColumns(self):
    from jpa_export_helpers import Columns
    return Columns.getPrimaryKeyColumns(self.table.columns)

Note, I'm originally a Java guy so importing "at will" seems strange to me. Anyway, what's the problem here?

---

When you import into a module, you import into the module's namespace. Thus when you have

 from jpa_export_helpers import SourceFile

in plugin_jrt_py you have actually created a name plugin_jrt_py.SourceFile. Following namespace resolution, within plug_in_jrt.py that name can be shortened to SourceFile but only within plug_in_jrt.

Because an import has side effects, the import statement is careful not to import a module twice.

You don't specify a calling sequence, but I suspect that fragments.py is imported by plugin_jrt.py and so the name is not accessible without qualification.

Try dropping the from clauses and the error will become more apparent.

---

Note how there is no error in the stacktrace related to anything other than importing. I found that sort of error almost always is related to recursive circular imports.