.. Copyright (C) 2001-2010 NLTK Project
.. For license information, see LICENSE.TXT

=========================================
 Loading Resources From the Data Package
=========================================

    >>> import nltk.data

Overview
~~~~~~~~
The `nltk.data` module contains functions that can be used to load
NLTK resource files, such as corpora, grammars, and saved processing
objects.

Loading Data Files
~~~~~~~~~~~~~~~~~~
Resources are loaded using the function `nltk.data.load()`, which
takes as its first argument a URL specifying what file should be
loaded.  The ``nltk:`` protocol loads files from the NLTK data
distribution:

    >>> tokenizer = nltk.data.load('nltk:tokenizers/punkt/english.pickle')
    >>> tokenizer.tokenize('Hello.  This is a test.  It works!')
    ['Hello.', 'This is a test.', 'It works!']

It is important to note that there should be no space following the
colon (':') in the URL; 'nltk: tokenizers/punkt/english.pickle' will
not work!

The ``nltk:`` protocol is used by default if no protocol is specified:

    >>> nltk.data.load('tokenizers/punkt/english.pickle') # doctest: +ELLIPSIS
    <nltk.tokenize.punkt.PunktSentenceTokenizer object at ...>

But it is also possible to load resources from ``http:``, ``ftp:``,
and ``file:`` URLs, e.g. ``cfg = nltk.data.load('http://example.com/path/to/toy.cfg')``

    >>> # Load a grammar using an absolute path.
    >>> url = 'file:%s' % nltk.data.find('grammars/sample_grammars/toy.cfg')
    >>> url # doctest: +ELLIPSIS
    'file:/.../nltk_data/grammars/sample_grammars/toy.cfg'
    >>> print nltk.data.load(url) # doctest: +ELLIPSIS
    Grammar with 14 productions (start state = S)
        S -> NP VP
        PP -> P NP
        ...
        P -> 'on'
        P -> 'in'

The second argument to the `nltk.data.load()` function specifies the
file format, which determines how the file's contents are processed
before they are returned by ``load()``.  The formats that are
currently supported by the data module are described by the dictionary
`nltk.data.FORMATS`:

    >>> for format, descr in sorted(nltk.data.FORMATS.items()):
    ...     print '%-7s %s' % (format, descr) # doctest: +NORMALIZE_WHITESPACE
    cfg     A context free grammar, parsed by nltk.parse_cfg().
    fcfg    A feature CFG, parsed by nltk.parse_fcfg().
    fol     A list of first order logic expressions, parsed by nltk.sem.parse_fol() using nltk.sem.logic.LogicParser.
    logic   A list of first order logic expressions, parsed by nltk.sem.parse_logic().  Requires an additional logic_parser parameter
    pcfg    A probabilistic CFG, parsed by nltk.parse_pcfg().
    pickle  A serialized python object, stored using the pickle module.
    raw     The raw (byte string) contents of a file.
    val     A semantic valuation, parsed by nltk.sem.parse_valuation().
    yaml    A serialized python object, stored using the yaml module.

`nltk.data.load()` will raise a ValueError if a bad format name is
specified:

    >>> nltk.data.load('grammars/sample_grammars/toy.cfg', 'bar')
    Traceback (most recent call last):
      . . .
    ValueError: Unknown format type!

By default, the ``"auto"`` format is used, which chooses a format
based on the filename's extension.  The mapping from file extensions
to format names is specified by `nltk.data.AUTO_FORMATS`:

    >>> for ext, format in sorted(nltk.data.AUTO_FORMATS.items()):
    ...     print '.%-7s -> %s' % (ext, format)
    .cfg     -> cfg
    .fcfg    -> fcfg
    .fol     -> fol
    .logic   -> logic
    .pcfg    -> pcfg
    .pickle  -> pickle
    .val     -> val
    .yaml    -> yaml

If `nltk.data.load()` is unable to determine the format based on the
filename's extension, it will raise a ValueError:

    >>> nltk.data.load('foo.bar')
    Traceback (most recent call last):
      . . .
    ValueError: Could not determine format for foo.bar based on its file
    extension; use the "format" argument to specify the format explicitly.

Note that by explicitly specifying the ``format`` argument, you can
override the load method's default processing behavior.  For example,
to get the raw contents of any file, simply use ``format="raw"``:

    >>> nltk.data.load('grammars/sample_grammars/toy.cfg', 'raw') # doctest: +ELLIPSIS
    "S -> NP VP\nPP -> P NP\nNP -> Det N | NP PP\nVP -> V NP | VP PP\n..."

Making Local Copies
~~~~~~~~~~~~~~~~~~~
..  This will not be visible in the html output: create a tempdir to
    play in.
    >>> import tempfile, os
    >>> tempdir = tempfile.mkdtemp()
    >>> old_dir = os.path.abspath('.')
    >>> os.chdir(tempdir)
    
The function `nltk.data.retrieve()` copies a given resource to a local
file.  This can be useful, for example, if you want to edit one of the
sample grammars.

    >>> nltk.data.retrieve('grammars/sample_grammars/toy.cfg')
    Retrieving 'grammars/sample_grammars/toy.cfg', saving to 'toy.cfg'
    
    >>> # Simulate editing the grammar.
    >>> s = open('toy.cfg').read().replace('NP', 'DP')
    >>> out = open('toy.cfg', 'w'); out.write(s); out.close()

    >>> # Load the edited grammar, & display it.
    >>> cfg = nltk.data.load('file:toy.cfg')
    >>> print cfg # doctest: +ELLIPSIS
    Grammar with 14 productions (start state = S)
        S -> DP VP
        PP -> P DP
        ...
        P -> 'on'
        P -> 'in'

The second argument to `nltk.data.retrieve()` specifies the filename
for the new copy of the file.  By default, the source file's filename
is used.

    >>> nltk.data.retrieve('grammars/sample_grammars/toy.cfg', 'mytoy.cfg')
    Retrieving 'grammars/sample_grammars/toy.cfg', saving to 'mytoy.cfg'
    >>> os.path.isfile('./mytoy.cfg')
    True
    >>> nltk.data.retrieve('grammars/sample_grammars/np.fcfg')
    Retrieving 'grammars/sample_grammars/np.fcfg', saving to 'np.fcfg'
    >>> os.path.isfile('./np.fcfg')
    True

If a file with the specified (or default) filename already exists in
the current directory, then `nltk.data.retrieve()` will raise a
ValueError exception.  It will *not* overwrite the file:

    >>> os.path.isfile('./toy.cfg')
    True
    >>> nltk.data.retrieve('grammars/sample_grammars/toy.cfg') # doctest: +ELLIPSIS
    Traceback (most recent call last):
      . . .
    ValueError: File '.../toy.cfg' already exists!

..  This will not be visible in the html output: clean up the tempdir.
    >>> os.chdir(old_dir)
    >>> for f in os.listdir(tempdir):
    ...     os.remove(os.path.join(tempdir, f))
    >>> os.rmdir(tempdir)

Finding Files in the NLTK Data Package
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The `nltk.data.find()` function searches the NLTK data package for a
given file, and returns a pointer to that file.  This pointer can
either be a `FileSystemPathPointer` (whose `path` attribute gives the
absolute path of the file); or a `ZipFilePathPointer`, specifying a
zipfile and the name of an entry within that zipfile.  Both pointer
types define the `open()` method, which can be used to read the string
contents of the file.

    >>> path = nltk.data.find('corpora/abc/rural.txt')
    >>> str(path).endswith('/nltk_data/corpora/abc/rural.txt') # doctest: +ELLIPSIS
    True
    >>> path.open().read(60)
    'PM denies knowledge of AWB kickbacks\nThe Prime Minister has '

Alternatively, the `nltk.data.load()` function can be used with the
keyword argument ``format="raw"``:

    >>> nltk.data.load('corpora/abc/rural.txt', format='raw')[:60]
    'PM denies knowledge of AWB kickbacks\nThe Prime Minister has '

Resource Caching
~~~~~~~~~~~~~~~~

NLTK uses a weakref dictionary to maintain a cache of resources that
have been loaded.  If you load a resource that is already stored in
the cache, then the cached copy will be returned.  This behavior can
be seen by the trace output generated when verbose=True:

    >>> feat0 = nltk.data.load('grammars/book_grammars/feat0.fcfg', verbose=True)
    <<Loading grammars/book_grammars/feat0.fcfg>>
    >>> feat0 = nltk.data.load('grammars/book_grammars/feat0.fcfg', verbose=True)
    <<Using cached copy of grammars/book_grammars/feat0.fcfg>>

If you wish to load a resource from its source, bypassing the cache,
use the ``cache=False`` argument to `nltk.data.load()`.  This can be
useful, for example, if the resource is loaded from a local file, and
you are actively editing that file:

    >>> feat0 = nltk.data.load('grammars/book_grammars/feat0.fcfg',cache=False,verbose=True)
    <<Loading grammars/book_grammars/feat0.fcfg>>

The cache *no longer* uses weak references.  A resource will not be
automatically expunged from the cache when no more objects are using
it.  In the following example, when we clear the variable ``feat0``,
the reference count for the feature grammar object drops to zero.
However, the object remains cached:

    >>> del feat0
    >>> feat0 = nltk.data.load('grammars/book_grammars/feat0.fcfg',
    ...                        verbose=True)
    <<Using cached copy of grammars/book_grammars/feat0.fcfg>>

You can clear the entire contents of the cache, using
`nltk.data.clear_cache()`:

    >>> nltk.data.clear_cache()

Retrieving other Data Sources
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    >>> formulas = nltk.data.load('grammars/book_grammars/background.fol')
    >>> for f in formulas: print str(f)
    all x.(boxerdog(x) -> dog(x))
    all x.(boxer(x) -> person(x))
    all x.-(dog(x) & person(x))
    all x.(married(x) <-> exists y.marry(x,y))
    all x.(bark(x) -> dog(x))
    all x y.(marry(x,y) -> (person(x) & person(y)))
    -(Vincent = Mia)
    -(Vincent = Fido)
    -(Mia = Fido)

Regression Tests
~~~~~~~~~~~~~~~~
Create a temp dir for tests that write files:

    >>> import tempfile, os
    >>> tempdir = tempfile.mkdtemp()
    >>> old_dir = os.path.abspath('.')
    >>> os.chdir(tempdir)

The `retrieve()` function accepts all url types:

    >>> urls = ['http://nltk.googlecode.com/svn/trunk/nltk/nltk/test/toy.cfg',
    ...         'file:%s' % nltk.data.find('grammars/sample_grammars/toy.cfg'),
    ...         'nltk:grammars/sample_grammars/toy.cfg',
    ...         'grammars/sample_grammars/toy.cfg']
    >>> for i, url in enumerate(urls):
    ...     nltk.data.retrieve(url, 'toy-%d.cfg' % i) # doctest: +ELLIPSIS
    Retrieving 'http://nltk.googlecode.com/svn/trunk/nltk/nltk/test/toy.cfg', saving to 'toy-0.cfg'
    Retrieving 'file:/.../nltk_data/grammars/sample_grammars/toy.cfg', saving to 'toy-1.cfg'
    Retrieving 'nltk:grammars/sample_grammars/toy.cfg', saving to 'toy-2.cfg'
    Retrieving 'grammars/sample_grammars/toy.cfg', saving to 'toy-3.cfg'

Clean up the temp dir:

    >>> os.chdir(old_dir)
    >>> for f in os.listdir(tempdir):
    ...     os.remove(os.path.join(tempdir, f))
    >>> os.rmdir(tempdir)

.. note:: load(..., format='yaml') is not currently tested, because we
    do not have any yaml files in our data distribution yet.  We should
    probably have a trained brill tagger there.

Lazy Loader
-----------
A lazy loader is a wrapper object that defers loading a resource until
it is accessed or used in any way.  This is mainly intended for
internal use by NLTK's corpus readers.

    >>> # Create a lazy loader for toy.cfg.
    >>> ll = nltk.data.LazyLoader('grammars/sample_grammars/toy.cfg')

    >>> # Show that it's not loaded yet:
    >>> object.__repr__(ll) # doctest: +ELLIPSIS
    '<nltk.data.LazyLoader object at ...>'
    
    >>> # printing it is enough to cause it to be loaded:
    >>> print ll
    <Grammar with 14 productions>

    >>> # Show that it's now been loaded:
    >>> object.__repr__(ll) # doctest: +ELLIPSIS
    '<nltk.grammar.ContextFreeGrammar object at ...>'


    >>> # Test that accessing an attribute also loads it:
    >>> ll = nltk.data.LazyLoader('grammars/sample_grammars/toy.cfg')
    >>> ll.start()
    S
    >>> object.__repr__(ll) # doctest: +ELLIPSIS
    '<nltk.grammar.ContextFreeGrammar object at ...>'

