From NLTK

NLTK developers should link to external packages according to the following guidelines:

• define a config_PACKAGE() function that can be used to explicitly state where PACKAGE binaries are installed (and any other relevant information)
• if this function is called with no arguments, then it will search for PACKAGE in a reasonable set of default locations, possibly including locations taken from environment variables.
• if this function can't find an installation of PACKAGE, then it raises an exception.
• if a Python interface function to PACKAGE is called before config_PACKAGE() has been called, then config_PACKAGE() will be called automatically with no arguments.