From NLTK

NLTK Packages conform to the following structure:

all modules

• opening comment with package name, copyright, license, authors

__init__.py

• package-level docstring
• import statements that import module names (but not api or util) into the package namespace: from module import * (for all modules in the package)
• define __all__
• mappings from deprecated functions to current functions, with deprecation decorators
• example: http://nltk.org/nltk/chunk/__init__.py

api.py

• define any interface classes that are used in other modules
• this should be imported into other modules
• (this filename is misleading as it is not an external API)
• example: http://nltk.org/nltk/chunk/api.py

util.py

• common utilities shared by modules in this package
• example: http://nltk.org/nltk/chunk/util.py

module.py

• imports in three blocks: standard library and third party imports, nltk imports, package-local imports
• imports from this package are local imports, of the form from other_module import name (and not from nltk.package.other_module import name; to facilitate moving, splitting and merging of packages)
• imports from other nltk packages are fully qualified, e.g. from nltk import tokenize.