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Package Definition

In order for Python to make use of a directory as package, the directory must have a name that is a valid Python identifier and contain a special module named __init__. Valid Python names are composed of letters, digits and underscores. See the section called “Variables” for more informaiton.

The __init__ module is often an empty file, in the package directory. Nothing else is required to make a directory into a package. The file, however, is essential. Without it, you'll get an ImportError.

For example, consider a number of modules related to the definition of cards. We might have the following files.


The cards.standard module would provide the base definition of card as an object with suit and rank. The cards.blackjack module would provide the subclasses of cards that we looked at in the section called “Blackjack Hands”. The module would provided the subclasses of cards that we looked at in the section called “Poker Hands”.

The cards.blackjack module and the module should both import the cards.standard module to get the base definition for the Card and Deck classes.

The __init__ module. The __init__ module is the "initialization" module in a package. It is processed the first time that a package name is encountered in an import statement. In effect, it initializes Python's understanding of the package. Since it is always loaded, it is also effectively the default module in a package. There are a number of consequences to this.

  • We can import the package, without naming a specific module. In this case we've imported just the initialization module, __init__. If this is part of our design, we'll put some kind of default or top-level definitions in the __init__ module.

  • We can import a specific module from the package. In this case, we also import the initialization module along with the requested module. In this case, the __init__ module can provide additional definitions; or it can simply be empty.

In our cards example, above, we would do well to make the __init__ module define the basic Card and Deck classes. If we import the package, cards, we get the default module, __init__, which gives us cards.Card and cards.Deck. If we import a specific module like cards.blackjack, we get the __init__ module plus the named module within the package.

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