4. attributes

Last page update: November 10, 2011

• Setting attributes with setItem method
• Setting attributes with setAttr method
• Getting attributes
• Attributes in a path: insert a condition

You can attach metadatas to any node of a Bag. Let’s introduce the attributes: each attribute has a name and a value and they are stored in a dictionary.

4.1. Setting attributes with setItem method

You can set attributes while you set an item, passing them as **kwargs of the setItem() method:

>>> b=Bag()
>>> b.setItem('documents.letters.letter_to_mark','file0',createdOn='10-7-2003',createdBy= 'Jack')
>>> b.setItem('documents.letters.letter_to_john','file1',createdOn='11-5-2003',createdBy='Mark',lastModify='11-9-2003')
>>> b.setItem('documents.letters.letter_to_sheila','file2')
>>> print b
0 - (Bag) documents:
    0 - (Bag) letters:
        0 - (str) letter_to_mark: file0  <createdOn='10-7-2003' createdBy='Jack'>
        1 - (str) letter_to_john: file1  <lastModify='11-9-2003' createdOn='11-5-2003' createdBy='Mark'>
        2 - (str) letter_to_sheila: file2

4.2. Setting attributes with setAttr method

With setAttr() method you can set, modify or delete attributes. The attributes are passed as **kwargs; let’s add some attributes to the letter to Sheila (the Bag item labelled “2”):

>>> b.setAttr('documents.letters.letter_to_sheila', createdOn='12-4-2003', createdBy='Walter', lastModify= '12-9-2003')
>>> b.setAttr('documents.letters.letter_to_sheila', fileOwner='Steve')
>>> print b
0 - (Bag) documents:
    0 - (Bag) letters:
        0 - (str) letter_to_mark: file0  <createdOn='10-7-2003' createdBy='Jack'>
        1 - (str) letter_to_john: file1  <lastModify='11-9-2003' createdOn='11-5-2003' createdBy='Mark'>
        2 - (str) letter_to_sheila: file2  <lastModify='12-9-2003' createdOn='12-4-2003' fileOwner='Steve' createdBy='Walter'>

You may delete an attribute assigning None to an existing value:

>>> b.setAttr('documents.letters.letter_to_sheila', fileOwner=None)
>>> print b
0 - (Bag) documents:
    0 - (Bag) letters:
        0 - (str) letter_to_sheila: file2  <lastModify='12-9-2003' createdOn='12-4-2003' createdBy='Walter'>

4.3. Getting attributes

To get a single Bag’s attribute you can use the getAttr() method:

>>> print b.getAttr('documents.letters.letter_to_sheila', 'fileOwner')
Steve

Square-brackets notations: you have to use the special char ? followed by the attribute’s name:

>>> print b['documents.letters.letter_to_sheila?fileOwner']
Steve

4.4. Attributes in a path: insert a condition

If the attribute value is a string type, there is another way [1] to identify a step of a bag path: by specifying a condition on any of its attributes.

The syntax for testing a condition on an attribute within a path is: #attributeName=value. Let’s check an example:

>>> bookcase = Bag()
>>> mybook=Bag()
>>> mybook.setItem('part1',Bag(),title='The fellowship of the ring',pages=213)
>>> mybook.setItem('part2',Bag(),title='The two towers',pages=221)
>>> mybook.setItem('part3',Bag(),title='The return of the king',pages=242)
>>> bookcase.setItem('genres.fantasy.LOTR',mybook,title='the Lord Of The Rings',id='f123',author='Tolkien')

If we print now our bookcase, we’ll get this:

>>> print bookcase
0 - (Bag) genres:
    0 - (Bag) fantasy:
        0 - (Bag) LOTR: <author='Tolkien' id='f123' title='the Lord Of The Rings'>
            0 - (Bag) part1: <pages='213' title='The fellowship of the ring'>
            1 - (Bag) part2: <pages='221' title='The two towers'>
            2 - (Bag) part3: <pages='242' title='The return of the king'>

Now, if we want to get the information of the LOTR Bag, we can use one of the these following lines:

>>> print bookcase.getItem('genres.fantasy.#author=Tolkien')
>>> print bookcase.getItem('genres.fantasy.LOTR')
>>> print bookcase.getItem('genres.fantasy.#=f123')

The result will be always the same:

0 - (Bag) part1: <pages='213' title='The fellowship of the ring'>
1 - (Bag) part2: <pages='221' title='The two towers'>
2 - (Bag) part3: <pages='242' title='The return of the king'>

You can omit the attribute’s name if the attribute is called id, but remember that this syntax works only if the attribute has a value of type string:

>>> print bookcase.getAttr('genres.fantasy.#=f123', 'title')
the Lord Of The Rings

Footnotes:

[1]	The other two ways are: labels (check the Basic functions introduction section) and numeric index #index (check the Getting Values (advanced) section).