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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN""http://www.w3.org/TR/html4/loose.dtd">
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files inside PyTables</TITLE
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CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
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TITLE="PyTables User's Guide"
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HREF="index.html"><LINK
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TITLE="FileNode - simulating a filesystem with PyTables"
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HREF="c5708.html"><LINK
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TITLE="Finding a FileNode
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HREF="x5744.html"><LINK
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TITLE="Complementary notes"
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HREF="x5875.html"></HEAD
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SUMMARY="Header navigation table"
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> User's Guide: Hierarchical datasets in Python - Release 1.3.2</TH
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>Chapter 6. FileNode - simulating a filesystem with <SPAN
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> sub-package of <SAMP
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The recommended way to import the module is:
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> >>> from tables.nodes import FileNode
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few symbols, so you can import <SAMP
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interactive usage. In fact, you will most probably only use
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> constant contains the value that
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> system attribute of a node file is expected
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>, as we have seen). Although
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this is not expected to change, you should use <SPAN
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>FileNode.NodeType</SPAN
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equivalent to the Python <SAMP
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>) for ordinary files. Their arguments
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differ from that of <SAMP
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only point where you will note the difference between
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working with a node file and working with an ordinary file.
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>For this little tutorial, we will assume that we have a
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> database opened for writing. Also, if
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you are somewhat lazy at typing sentences, the code that we
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are going to explain is included in the
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>examples/filenodes1.py</SAMP
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>You can create a brand new file with these sentences:
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> >>> import tables
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>>> h5file = tables.openFile('fnode.h5', 'w')
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NAME="subsection6.3.1"
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>6.3.1. Creating a new file node</A
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>Creation of a new file node is achieved with the
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> call. You must tell it in which
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> file you want to create it, where in
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> hierarchy you want to create the
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node and which will be its name. The
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> file is the first argument to
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>; it will be also called the
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>'host PyTables file'</SAMP
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arguments must be given as keyword arguments
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As a result of the call, a brand new appendable and
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readable file node object is returned.
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>So let us create a new node file in the previously opened
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> and placed right under the root
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of the database hierarchy. This is that command:
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> >>> fnode = FileNode.newNode(h5file, where='/', name='fnode_test')
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>That is basically all you need to create a file node.
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Simple, isn't it? From that point on, you can use
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> as any opened Python file (i.e. you can
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write data, read data, lines of text and so on).
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> accepts some more keyword
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arguments. You can give a title to your file with the
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> argument. You can use
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>' compression features with the
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> argument. If you know beforehand the
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size that your file will have, you can give its final file
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size in bytes to the <SAMP
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> library would be able to
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optimize the data access.
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node where it is told to. To prove it, we will try to get
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> attribute from the newly created
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> >>> print h5file.getNodeAttr('/fnode_test', 'NODE_TYPE')
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NAME="subsection6.3.2"
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>6.3.2. Using a file node</A
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>As stated above, you can use the new node file as any
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other opened file. Let us try to write some text in and
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> >>> print >> fnode, "This is a test text line."
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>>> print >> fnode, "And this is another one."
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>>> print >> fnode
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>>> fnode.write("Of course, file methods can also be used.")
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>>> fnode.seek(0) # Go back to the beginning of file.
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>>> for line in fnode:
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'This is a test text line.\n'
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'And this is another one.\n'
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'Of course, file methods can also be used.'
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>This was run on a Unix system, so newlines are expressed
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>. In fact, you can override the line
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separator for a file by setting its
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> property to any string you
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>While using a file node, you should take care of closing
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> host file. Because of the way
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> works, your data it will not be at a
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risk, but every operation you execute after closing the
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host file will fail with a <SAMP
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close a file node, simply delete the corresponding reference
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> >>> fnode.close()
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>>> print fnode.closed
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NAME="subsection6.3.3"
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>6.3.3. Opening an existing file node</A
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>If you have a file node that you created using
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>, you can open it later by calling
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>. Its arguments are similar to
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first argument is the <SAMP
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want to open (i.e. a node with a <SAMP
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attribute having a <SAMP
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second argument is a mode string indicating how to open
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the file. Contrary to <SAMP
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> can not be used to create a new
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>File nodes can be opened in read-only mode
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>) or in read-and-append mode
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>). Reading from a file node is allowed
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in both modes, but appending is only allowed in the second
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one. Just like Python files do, writing data to an
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appendable file places it after the file pointer if it is
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on or beyond the end of the file, or otherwise after the
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existing data. Let us see an example:
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> >>> node = h5file.root.fnode_test
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>>> fnode = FileNode.openNode(node, 'a+')
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>>> print repr(fnode.readline())
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'This is a test text line.\n'
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>>> print fnode.tell()
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>>> print >> fnode, "This is a new line."
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>>> print repr(fnode.readline())
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>Of course, the data append process places the pointer at
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the end of the file, so the last <SAMP
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>. Let us seek to the beginning
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of the file to see the whole contents of our file.
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> >>> fnode.seek(0)
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>>> for line in fnode:
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'This is a test text line.\n'
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'And this is another one.\n'
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'Of course, file methods can also be used.This is a new line.\n'
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>As you can check, the last string we wrote was correctly
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appended at the end of the file, instead of overwriting
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the second line, where the file pointer was positioned by
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the time of the appending.
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NAME="subsection6.3.4"
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>6.3.4. Adding metadata to a file node</A
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>You can associate arbitrary metadata to any open node
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file, regardless of its mode, as long as the host
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> file is writable. Of course, you
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> to do it directly on the proper
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much more comfortable way to do it. <SAMP
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objects have an <SAMP
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> property which gives
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you direct access to their corresponding
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>For instance, let us see how to associate MIME type
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metadata to our file node:
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> >>> fnode.attrs.content_type = 'text/plain; charset=us-ascii'
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>As simple as A-B-C. You can put nearly anything in an
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attribute, which opens the way to authorship, keywords,
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permissions and more. Moreover, there is not a fixed list
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of attributes. However, you should avoid names in all
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caps or starting with <SAMP
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> may use them internally.
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> >>> fnode.attrs.author = "Ivan Vilata i Balaguer"
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>>> fnode.attrs.creation_date = '2004-10-20T13:25:25+0200'
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>>> fnode.attrs.keywords_en = ["FileNode", "test", "metadata"]
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>>> fnode.attrs.keywords_ca = ["FileNode", "prova", "metadades"]
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>>> fnode.attrs.owner = 'ivan'
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>>> fnode.attrs.acl = {'ivan': 'rw', '@users': 'r'}
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>You can check that these attributes get stored by running
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> command on the host
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> $ ptdump -a fnode.h5:/fnode_test
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/fnode_test (EArray(113,)) ''
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/fnode_test.attrs (AttributeSet), 14 attributes:
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FLAVOR := 'numarray',
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NODE_TYPE_VERSION := 2,
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acl := {'ivan': 'rw', '@users': 'r'},
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author := 'Ivan Vilata i Balaguer',
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content_type := 'text/plain; charset=us-ascii',
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creation_date := '2004-10-20T13:25:25+0200',
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keywords_ca := ['FileNode', 'prova', 'metadades'],
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keywords_en := ['FileNode', 'test', 'metadata'],
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assumptions about the meaning of your metadata, so its
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handling is entirely left to your needs and imagination.
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SUMMARY="Footer navigation table"
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>Complementary notes</TD
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