V (Vgroup) API (`pyhdf.V`)¶

A module of the pyhdf package implementing the V (Vgroup) API of the NCSA HDF4 library.

Introduction¶

V is one of the modules composing pyhdf, a python package implementing the NCSA HDF library and letting one manage HDF files from within a python program. Two versions of the HDF library currently exist, version 4 and version 5. pyhdf only implements version 4 of the library. Many different APIs are to be found inside the HDF4 specification. Currently, pyhdf implements just a few of those: the SD, VS and V APIs. Other APIs should be added in the future (GR, AN, etc).

The V API supports the definition of vgroups inside an HDF file. A vgroup can thought of as a collection of arbitrary “references” to other HDF objects defined in the same file. A vgroup may hold references to other vgroups. It is thus possible to organize HDF objects into some sort of a hierarchy, similar to files grouped into a directory tree under unix. This vgroup hierarchical nature partly explains the origin of the “HDF” name (Hierarchical Data Format). vgroups can help logically organize the contents of an HDF file, for example by grouping together all the datasets belonging to a given experiment, and subdividing those datasets according to the day of the experiment, etc.

The V API provides functions to find and access an existing vgroup, create a new one, delete a vgroup, identify the members of a vgroup, add and remove members to and from a vgroup, and set and query attributes on a vgroup. The members of a vgroup are identified through their tags and reference numbers. Tags are constants identifying each main object type (dataset, vdata, vgroup). Reference numbers serve to distinguish among objects of the same type. To add an object to a vgroup, one must first initialize that object using the API proper to that object type (eg: SD for a dataset) so as to create a reference number for that object, and then pass this reference number and the type tag to the V API. When reading the contents of a vgroup, the V API returns the tags and reference numbers of the objects composing the vgroup. The user program must then call the proper API to process each object, based on tag of this object (eg: VS for a tag identifying a vdata object).

Some limitations of the V API must be stressed. First, HDF imposes no integrity constraint whatsoever on the contents of a vgroup, nor does it help maintain such integrity. For example, a vgroup is not strictly hierarchical, because an object can belong to more than one vgroup. It would be easy to create vgroups showing cycles among their members. Also, a vgroup member is simply a reference to an HDF object. If this object is afterwards deleted for any reason, the vgroup membership will not be automatically updated. The vgroup will refer to a non-existent object and thus be left in an inconsistent state. Nothing prevents adding the same member more than once to a vgroup, and giving the same name to more than one vgroup. Finally, the HDF library seems to make heavy use of vgroups for its own internal needs, and creates vgroups “behind the scenes”. This may make it difficult to pick up “user defined” vgroups when browsing an HDF file.

Accessing the V module¶

To access the V module a python program can say one of:

>>> import pyhdf.V           # must prefix names with "pyhdf.V."
>>> from pyhdf import V      # must prefix names with "V."
>>> from pyhdf.V import *    # names need no prefix

This document assumes the last import style is used.

V is not self-contained, and needs functionality provided by another pyhdf module, namely the HDF module. This module must thus be imported also:

>>> from .HDF import *

Package components¶

pyhdf is a proper Python package, eg a collection of modules stored under a directory whose name is that of the package and which stores an __init__.py file. Following the normal installation procedure, this directory will be <python-lib>/site-packages/pyhdf’, where <python-lib> stands for the python installation directory.

For each HDF API exists a corresponding set of modules.

The following modules are related to the V API.

_hdfext
C extension module responsible for wrapping the HDF C library for all python modules

hdfext
python module implementing some utility functions complementing the _hdfext extension module

error
defines the HDF4Error exception

HDF
python module providing support to the V module

V
python module wrapping the V API routines inside an OOP framework

_hdfext and hdfext were generated using the SWIG preprocessor. SWIG is however not needed to run the package. Those two modules are meant to do their work in the background, and should never be called directly. Only HDF and V should be imported by the user program.

Prerequisites¶

The following software must be installed in order for the V module to work.

HDF (v4) library
pyhdf does not include the HDF4 library, which must be installed separately.

HDF is available at: “https://portal.hdfgroup.org/display/support/Download+HDF4”.

Numeric is also needed by the SD module. See the SD module documentation.

Summary of differences between the pyhdf and C V API¶

Most of the differences between the pyhdf and C V API can be summarized as follows.

-In the C API, every function returns an integer status code, and values
computed by the function are returned through one or more pointers passed as arguments.

-In pyhdf, error statuses are returned through the Python exception
mechanism, and values are returned as the method result. When the C API specifies that multiple values are returned, pyhdf returns a sequence of values, which are ordered similarly to the pointers in the C function argument list.

Error handling¶

All errors reported by the C V API with a SUCCESS/FAIL error code are reported by pyhdf using the Python exception mechanism. When the C library reports a FAIL status, pyhdf raises an HDF4Error exception (a subclass of Exception) with a descriptive message. Unfortunately, the C library is rarely informative about the cause of the error. pyhdf does its best to try to document the error, but most of the time cannot do more than saying “execution error”.

V needs support from the HDF module¶

The VS module is not self-contained (countrary to the SD module). It requires help from the HDF module, namely:

the HDF.HDF class to open and close the HDF file, and initialize the V interface
the HDF.HC class to provide different sorts of constants (opening modes, data types, etc).

A program wanting to access HDF vgroups will almost always need to execute the following minimal set of calls:

>>> from pyhdf.HDF import *
>>> from pyhdf.V import *
>>> hdfFile = HDF(name, HC.xxx)# open HDF file
>>> v = hdfFile.vgstart()      # initialize V interface on HDF file
>>> ...                        # manipulate vgroups
>>> v.end()                    # terminate V interface
>>> hdfFile.close()            # close HDF file

Classes summary¶

pyhdf wraps the V API using the following python classes:

V      HDF V interface
VG     vgroup
VGAttr vgroup attribute

In more detail:

V     The V class implements the V (Vgroup) interface applied to an
      HDF file.

      To instantiate a V class, call the vgstart() method of an
      HDF instance.

      methods:
        constructors
          attach()      open an existing vgroup given its name or its
                        reference number, or create a new vgroup,
                        returning a VG instance for that vgroup
          create()      create a new vgroup, returning a VG instance
                        for that vgroup

        closing the interface
          end()         close the V interface on the HDF file

        deleting a vgroup
          delete()      delete the vgroup identified by its name or
                        its reference number

        searching
          find()        find a vgroup given its name, returning
                        the vgroup reference number
          findclass()   find a vgroup given its class name, returning
                        the vgroup reference number
          getid()       return the reference number of the vgroup
                        following the one with the given reference number

VG    The VG class encapsulates the functionality of a vgroup.

      To instantiate a VG class, call the attach() or create() methods
      of a V class instance.

        constructors

          attr()        return a VGAttr instance representing an attribute
                        of the vgroup
          findattr()    search the vgroup for a given attribute,
                        returning a VGAttr instance for that attribute

        ending access to a vgroup

          detach()      terminate access to the vgroup

        adding a member to a vgroup

          add()         add to the vgroup the HDF object identified by its
                        tag and reference number
          insert()      insert a vdata or a vgroup in the vgroup, given
                        the vdata or vgroup instance

        deleting a member from a vgroup

          delete()      remove from the vgroup the HDF object identified
                        by the given tag and reference number

        querying vgroup

          attrinfo()    return info about all the vgroup attributes
          inqtagref()   determine if the HDF object with the given
                        tag and reference number belongs to the vgroup
          isvg()        determine if the member with the given reference
                        number is a vgroup object
          isvs()        determine if the member with the given reference
                        number is a vdata object
          nrefs()       return the number of vgroup members with the
                        given tag
          tagref()      get the tag and reference number of a vgroup
                        member, given the index number of that member
          tagrefs()     get the tags and reference numbers of all the
                        vgroup members

VGAttr The VGAttr class provides methods to set and query vgroup
       attributes.

       To create an instance of this class, call the attr() method
       of a VG instance.

       Remember that vgroup attributes can also be set and queried by
       applying the standard python "dot notation" on a VG instance.

         get attribute value(s)

           get()         obtain the attribute value(s)

         set attribute value(s)

           set()         set the attribute to the given value(s) of the
                         given type, first creating the attribute if
                         necessary

         query attribute info

           info()        retrieve attribute name, data type, order and
                         size

Attribute access: low and high level¶

The V API allows setting attributes on vgroups. Attributes can be of many types (int, float, char) of different bit lengths (8, 16, 32, 64 bits), and can be single or multi-valued. Values of a multi-valued attribute must all be of the same type.

Attributes can be set and queried in two different ways. First, given a VG instance (describing a vgroup object), the attr() method of that instance is called to create a VGAttr instance representing the wanted attribute (possibly non existent). The set() method of this VGAttr instance is then called to define the attribute value, creating it if it does not already exist. The get() method returns the current attribute value. Here is an example.

>>> from pyhdf.HDF import *
>>> from pyhdf.V import *
>>> f = HDF('test.hdf', HC.WRITE) # Open file 'test.hdf' in write mode
>>> v = f.vgstart()            # init vgroup interface
>>> vg = v.attach('vtest', 1)  # attach vgroup 'vtest' in write mode
>>> attr = vg.attr('version')  # prepare to define the 'version' attribute
                               # on the vdata
>>> attr.set(HC.CHAR8,'1.0')   # set attribute 'version' to string '1.0'
>>> print(attr.get())           # get and print attribute value
>>> attr = vg .attr('range')   # prepare to define attribute 'range'
>>> attr.set(HC.INT32,(-10, 15)) # set attribute 'range' to a pair of ints
>>> print(attr.get())             # get and print attribute value

>>> vg.detach()                # "close" the vgroup
>>> v.end()                    # terminate the vgroup interface
>>> f.close()                  # close the HDF file

The second way consists of setting/querying an attribute as if it were a normal python class attribute, using the usual dot notation. Above example then becomes:

>>> from pyhdf.HDF import *
>>> from pyhdf.V import *
>>> f = HDF('test.hdf', HC.WRITE) # Open file 'test.hdf' in write mode
>>> v = f.vgstart()            # init vgroup interface
>>> vg = v.attach('vtest', 1)  # attach vdata 'vtest' in write mode
>>> vg.version = '1.0'         # create vdata attribute 'version',
                               # setting it to string '1.0'
>>> print(vg.version)           # print attribute value
>>> vg.range = (-10, 15)       # create attribute 'range', setting
                               # it to the pair of ints (-10, 15)
>>> print(vg.range)             # print attribute value
>>> vg.detach()                # "close" the vdata
>>> v.end()                    # terminate the vdata interface
>>> f.close()                  # close the HDF file

Note how the dot notation greatly simplifies and clarifies the code. Some latitude is however lost by manipulating attributes in that way, because the pyhdf package, not the programmer, is then responsible of setting the attribute type. The attribute type is chosen to be one of:

HC.CHAR8

if the attribute value is a string

HC.INT32

if all attribute values are integers

HC.FLOAT64

otherwise

The first way of handling attribute values must be used if one wants to define an attribute of any other type (for ex. 8 or 16 bit integers, signed or unsigned). Also, only a VDAttr instance gives access to attribute info, through its info() method.

However, accessing HDF attributes as if they were python attributes raises an important issue. There must exist a way to assign generic attributes to the python objects without requiring those attributes to be converted to HDF attributes. pyhdf uses the following rule: an attribute whose name starts with an underscore (‘_’) is either a “predefined” HDF attribute (see below) or a standard python attribute. Otherwise, the attribute is handled as an HDF attribute. Also, HDF attributes are not stored inside the object dictionary: the python dir() function will not list them.

Attribute values can be updated, but it is illegal to try to change the value type, or the attribute order (number of values). This is important for attributes holding string values. An attribute initialized with an ‘n’ character string is simply a character attribute of order ‘n’ (eg a character array of length ‘n’). If ‘vg’ is a vgroup and we initialize its ‘a1’ attribute as ‘vg.a1 = “abcdef”’, then a subsequent update attempt like ‘vg.a1 = “12”’ will fail, because we then try to change the order of the attribute (from 6 to 2). It is mandatory to keep the length of string attributes constant.

Predefined attributes¶

The VG class supports predefined attributes to get (and occasionnaly set) attribute values easily using the usual python “dot notation”, without having to call a class method. The names of predefined attributes all start with an underscore (‘_’).

In the following table, the RW column holds an X if the attribute is read/write.

VG predefined attributes

name

RW

description

C library routine

_class

X

class name

Vgetclass/Vsetclass

_name

X

vgroup name

Vgetname/Vsetname

_nattrs

number of vgroup attributes

Vnattrs

_nmembers

number of vgroup members

Vntagrefs

_refnum

vgroup reference number

VQueryref

_tag

vgroup tag

VQuerytag

_version

vgroup version number

Vgetversion

Programming models¶

Creating and initializing a vgroup¶

The following program shows how to create and initialize a vgroup inside an HDF file. It can serve as a model for any program wanting to create a vgroup:

from pyhdf.HDF import *
from pyhdf.V   import *
from pyhdf.VS  import *
from pyhdf.SD  import *

def vdatacreate(vs, name):

    # Create vdata and define its structure
    vd = vs.create(name,
               (('partid',HC.CHAR8, 5),       # 5 char string
                ('description',HC.CHAR8, 10), # 10 char string field
                ('qty',HC.INT16, 1),          # 1 16 bit int field
                ('wght',HC.FLOAT32, 1),       # 1 32 bit float
                ('price',HC.FLOAT32,1)        # 1 32 bit float
               ))

    # Store records
    vd.write((('Q1234', 'bolt',12, 0.01, 0.05),   # record 1
              ('B5432', 'brush', 10, 0.4, 4.25),  # record 2
              ('S7613', 'scissor', 2, 0.2, 3.75)  # record 3
              ))
    # "close" vdata
    vd.detach()

def sdscreate(sd, name):

    # Create a simple 3x3 float array.
    sds = sd.create(name, SDC.FLOAT32, (3,3))
    # Initialize array
    sds[:] = ((0,1,2),(3,4,5),(6,7,8))
    # "close" dataset.
    sds.endaccess()

# Create HDF file
filename = 'inventory.hdf'
hdf = HDF(filename, HC.WRITE|HC.CREATE)

# Initialize the SD, V and VS interfaces on the file.
sd = SD(filename, SDC.WRITE)  # SD interface
vs = hdf.vstart()             # vdata interface
v  = hdf.vgstart()            # vgroup interface

# Create vdata named 'INVENTORY'.
vdatacreate(vs, 'INVENTORY')
# Create dataset named "ARR_3x3"
sdscreate(sd, 'ARR_3x3')

# Attach the vdata and the dataset.
vd = vs.attach('INVENTORY')
sds = sd.select('ARR_3x3')

# Create vgroup named 'TOTAL'.
vg = v.create('TOTAL')

# Add vdata to the vgroup
vg.insert(vd)
# We could also have written this:
# vgroup.add(vd._tag, vd._refnum)
# or this:
# vgroup.add(HC.DFTAG_VH, vd._refnum)

# Add dataset to the vgroup
vg.add(HC.DFTAG_NDG, sds.ref())

# Close vgroup, vdata and dataset.
vg.detach()       # vgroup
vd.detach()       # vdata
sds.endaccess()   # dataset

# Terminate V, VS and SD interfaces.
v.end()           # V interface
vs.end()          # VS interface
sd.end()          # SD interface

# Close HDF file.
hdf.close()

The program starts by defining two functions vdatacreate() and sdscreate(), which will serve to create the vdata and dataset objects we need. Those functions are not essential to the example. They simply help to make the example self-contained. Refer to the VS and SD module documentation for additional explanations about how these functions work.

After opening the HDF file in write mode, the SD, V and VS interfaces are initialized on the file. Next vdatacreate() is called to create a new vdata named ‘INVENTORY’ on the VS instance, and sdscreate() to create a new dataset named ‘ARR_3x3’ on the SD instance. This is done so that we have a vdata and a dataset to play with.

The vdata and the dataset are then attached (“opened”). The create() method of the V instance is then called to create a new vgroup named ‘TOTAL’. The vgroup is then populated by calling its insert() method to add the vdata ‘INVENTORY’, and its add() method to add the ‘ARR_3x3’ dataset. Note that insert() is just a commodity method that simplifies adding a vdata or a vgroup to a vgroup, avoiding the need to pass an object tag and reference number. There is no such commodity method for adding a dataset to a vgroup. The dataset must be added by specifying its tag and reference number. Note that the tags to be used are defined inside the HDF module as constants of the HC class: DFTAG_NDG for a dataset, DFTAG_VG for a vgroup, DFTAG_VH for a vdata.

The program ends by detaching (“closing”) the HDF objects created above, terminating the three interfaces initialized, and closing the HDF file.

Reading a vgroup¶

The following program shows the contents of the vgroups contained inside any HDF file:

from pyhdf.HDF import *
from pyhdf.V   import *
from pyhdf.VS  import *
from pyhdf.SD  import *

import sys

def describevg(refnum):

    # Describe the vgroup with the given refnum.

    # Open vgroup in read mode.
    vg = v.attach(refnum)
    print "----------------"
    print "name:", vg._name, "class:",vg._class, "tag,ref:",
    print vg._tag, vg._refnum

    # Show the number of members of each main object type.
    print "members: ", vg._nmembers,
    print "datasets:", vg.nrefs(HC.DFTAG_NDG),
    print "vdatas:  ", vg.nrefs(HC.DFTAG_VH),
    print "vgroups: ", vg.nrefs(HC.DFTAG_VG)

    # Read the contents of the vgroup.
    members = vg.tagrefs()

    # Display info about each member.
    index = -1
    for tag, ref in members:
        index += 1
        print "member index", index
        # Vdata tag
        if tag == HC.DFTAG_VH:
            vd = vs.attach(ref)
            nrecs, intmode, fields, size, name = vd.inquire()
            print "  vdata:",name, "tag,ref:",tag, ref
            print "    fields:",fields
            print "    nrecs:",nrecs
            vd.detach()

        # SDS tag
        elif tag == HC.DFTAG_NDG:
            sds = sd.select(sd.reftoindex(ref))
            name, rank, dims, type, nattrs = sds.info()
            print "  dataset:",name, "tag,ref:", tag, ref
            print "    dims:",dims
            print "    type:",type
            sds.endaccess()

        # VS tag
        elif tag == HC.DFTAG_VG:
            vg0 = v.attach(ref)
            print "  vgroup:", vg0._name, "tag,ref:", tag, ref
            vg0.detach()

        # Unhandled tag
        else:
            print "unhandled tag,ref",tag,ref

    # Close vgroup
    vg.detach()

# Open HDF file in readonly mode.
filename = sys.argv[1]
hdf = HDF(filename)

# Initialize the SD, V and VS interfaces on the file.
sd = SD(filename)
vs = hdf.vstart()
v  = hdf.vgstart()

# Scan all vgroups in the file.
ref = -1
while 1:
    try:
        ref = v.getid(ref)
    except HDF4Error,msg:    # no more vgroup
        break
    describevg(ref)

# Terminate V, VS and SD interfaces.
v.end()
vs.end()
sd.end()

# Close HDF file.
hdf.close()

The program starts by defining function describevg(), which is passed the reference number of the vgroup to display. The function assumes that the SD, VS and V interfaces have been previously initialized.

The function starts by attaching (“opening”) the vgroup, and displaying its name, class, tag and reference number. The number of members of the three most important object types is then displayed, by calling the nrefs() method with the predefined tags found inside the HDF.HC class.

The tagrefs() method is then called to get a list of all the vgroup members, each member being identified by its tag and reference number. A ‘for’ statement is entered to loop over each element of this list. The tag is tested against the known values defined in the HDF.HC class: the outcome of this test indicates how to process the member object.

A DFTAG_VH tag indicates we deal with a vdata. The vdata is attached, its inquire() method called to display info about it, and the vdata is detached. In the case of a DFTAG_NFG, we are facing a dataset. The dataset is selected, info is obtained by calling the dataset info() method, and the dataset is released. A DFTAG_VG indicates that the member is a vgroup. We attach it, print its name, tag and reference number, then detach the member vgroup. A warning is finally displayed if we hit upon a member of an unknown type.

The function releases the vgroup just displayed and returns.

The main program starts by opening in readonly mode the HDF file passed as argument on the command line. The SD, VS and V interfaces are initialized, and the corresponding class instances are stored inside ‘sd’, ‘vs’ and ‘v’ global variables, respectively, for the use of the describevg() function.

A while loop is then entered to access each vgroup in the file. A reference number of -1 is passed on the first call to getid() to obtain the reference number of the first vgroup. getid() returns a new reference number on each subsequent call, and raises an exception when the last vgroup has been retrieved. This exception is caught to break out of the loop, otherwise describevg() is called to display the vgroup we have on hand.

Once the loop is over, the interfaces initialized before are terminated, and the HDF file is closed.

You will notice that this program will display vgroups other than those you have explicitly created. Those supplementary vgroups are created by the HDF library for its own internal needs.

class pyhdf.V.V(hinst)[source]¶

The V class implements the V (Vgroup) interface applied to an HDF file. To instantiate a V class, call the vgstart() method of an HDF instance.

attach(num_name, write=0)[source]¶

Open an existing vgroup given its name or its reference number, or create a new vgroup, returning a VG instance for that vgroup.

Args:

num_name      reference number or name of the vgroup to open,
              or -1 to create a new vgroup; vcreate() can also
              be called to create and name a new vgroup
write         set to non-zero to open the vgroup in write mode
              and to 0 to open it in readonly mode (default)

Returns:

VG instance for the vgroup

An exception is raised if an attempt is made to open a non-existent vgroup.

C library equivalent : Vattach

create(name)[source]¶

Create a new vgroup, and assign it a name.

Args:

name   name to assign to the new vgroup

Returns:

VG instance for the new vgroup

A create(name) call is equivalent to an attach(-1, 1) call, followed by a call to the setname(name) method of the instance.

C library equivalent : no equivalent

delete(num_name)[source]¶

Delete from the HDF file the vgroup identified by its reference number or its name.

Args:

num_name    either the reference number or the name of
            the vgroup to delete

Returns:

None

C library equivalent : Vdelete

end()[source]¶

Close the V interface.

Args:

No argument

Returns:

None

C library equivalent : Vend

find(name)[source]¶

Find a vgroup given its name, returning its reference number if found.

Args:

name     name of the vgroup to find

Returns:

vgroup reference number

An exception is raised if the vgroup is not found.

C library equivalent: Vfind

findclass(name)[source]¶

Find a vgroup given its class name, returning its reference number if found.

Args:

name     class name of the vgroup to find

Returns:

vgroup reference number

An exception is raised if the vgroup is not found.

C library equivalent: Vfind

getid(ref)[source]¶

Obtain the reference number of the vgroup following the vgroup with the given reference number .

Args:

ref    reference number of the vgroup after which to search;
       set to -1 to start the search at the start of
       the HDF file

Returns:

reference number of the vgroup past the one identified by 'ref'

An exception is raised if the end of the vgroup is reached.

C library equivalent : Vgetid

class pyhdf.V.VG(vinst, id)[source]¶

The VG class encapsulates the functionality of a vgroup. To instantiate a VG class, call the attach() or create() methods of a V class instance.

add(tag, ref)[source]¶

Add to the vgroup an object identified by its tag and reference number.

Args:

tag       tag of the object to add
ref       reference number of the object to add

Returns:

total number of objects in the vgroup after the addition

C library equivalent : Vaddtagref

attr(name_index)[source]¶

Create a VGAttr instance representing a vgroup attribute.

Args:

name_index  attribute name or attribute index number; if a
            name is given the attribute may not exist; in that
            case, it will be created when the VGAttr
            instance set() method is called

Returns:

VGAttr instance for the attribute. Call the methods of this
class to query, read or set the attribute.

C library equivalent : no equivalent

attrinfo()[source]¶

Return info about all the vgroup attributes.

Args:

no argument

Returns:

dictionary describing each vgroup attribute; for each attribute,
a (name,data) pair is added to the dictionary, where 'data' is
a tuple holding:

- attribute data type (one of HC.xxx constants)
- attribute order
- attribute value
- attribute size in bytes

C library equivalent : no equivalent

delete(tag, ref)[source]¶

Delete from the vgroup the member identified by its tag and reference number.

Args:

tag    tag of the member to delete
ref    reference number of the member to delete

Returns:

None

Only the link of the member with the vgroup is deleted. The member object is not deleted.

C library equivalent : Vdeletatagref

detach()[source]¶

Terminate access to the vgroup.

Args:

no argument

Returns:

None

C library equivalent : Vdetach

findattr(name)[source]¶

Search the vgroup for a given attribute.

Args:

name    attribute name

Returns:

 if found, VGAttr instance describing the attribute
 None otherwise

C library equivalent : Vfindattr

inqtagref(tag, ref)[source]¶

Determines if an object identified by its tag and reference number belongs to the vgroup.

Args:

tag      tag of the object to check
ref      reference number of the object to check

Returns:

False (0) if the object does not belong to the vgroup,
True  (1) otherwise

C library equivalent : Vinqtagref

insert(inst)[source]¶

Insert a vdata or a vgroup in the vgroup.

Args:

inst  vdata or vgroup instance to add

Returns:

index of the inserted vdata or vgroup (0 based)

C library equivalent : Vinsert

isvg(ref)[source]¶

Determines if the member of a vgoup is a vgroup.

Args:

ref      reference number of the member to check

Returns:

False (0) if the member is not a vgroup
True  (1) otherwise

C library equivalent : Visvg

isvs(ref)[source]¶

Determines if the member of a vgoup is a vdata.

Args:

ref      reference number of the member to check

Returns:

False (0) if the member is not a vdata,
True  (1) otherwise

C library equivalent : Visvs

nrefs(tag)[source]¶

Determine the number of tags of a given type in a vgroup.

Args:

tag    tag type to look for in the vgroup

Returns:

number of members identified by this tag type

C library equivalent : Vnrefs

tagref(index)[source]¶

Get the tag and reference number of a vgroup member, given the index number of that member.

Args:

index   member index (0 based)

Returns:

2-element tuple:
  - member tag
  - member reference number

C library equivalent : Vgettagref

tagrefs()[source]¶

Get the tags and reference numbers of all the vgroup members.

Args:

no argument

Returns:

list of (tag,ref) tuples, one for each vgroup member

C library equivalent : Vgettagrefs

class pyhdf.V.VGAttr(obj, name_or_index)[source]¶

The VGAttr class encapsulates methods used to set and query attributes defined on a vgroup. To create an instance of this class, call the attr() method of a VG class.

get()[source]¶

Retrieve the attribute value.

Args:

no argument

Returns:

attribute value(s); a list is returned if the attribute
is made up of more than one value, except in the case of a
string-valued attribute (data type HC.CHAR8) where the
values are returned as a string

Note that a vgroup attribute can also be queried like a standard python class attribute by applying the usual “dot notation” to a VG instance.

C library equivalent : Vgetattr

info()[source]¶

Retrieve info about the attribute.

Args:

no argument

Returns:

4-element tuple with the following components:
  -attribute name
  -attribute data type (one of HC.xxx constants)
  -attribute order (number of values)
  -attribute size in bytes

C library equivalent : Vattrinfo

set(data_type, values)[source]¶

Set the attribute value.

Args:

data_type    : attribute data type (see constants HC.xxx)
values       : attribute value(s); specify a list to create
               a multi-valued attribute; a string valued
               attribute can be created by setting 'data_type'
               to HC.CHAR8 and 'values' to the corresponding
               string

               If the attribute already exists, it will be
               updated. However, it is illegal to try to change
               its data type or its order (number of values).

Returns:

None

Note that a vgroup attribute can also be set like a standard python class attribute by applying the usual “dot notation” to a VG instance.

C library equivalent : Vsetattr

V (Vgroup) API (`pyhdf.V`)¶

Introduction¶

Accessing the V module¶

Package components¶

Prerequisites¶

Summary of differences between the pyhdf and C V API¶

Error handling¶

V needs support from the HDF module¶

Classes summary¶

Attribute access: low and high level¶

Predefined attributes¶

Programming models¶

Creating and initializing a vgroup¶

Reading a vgroup¶

Table of Contents

Previous topic

This Page

HC.CHAR8	if the attribute value is a string
HC.INT32	if all attribute values are integers
HC.FLOAT64	otherwise

name	RW	description	C library routine
_class	X	class name	Vgetclass/Vsetclass
_name	X	vgroup name	Vgetname/Vsetname
_nattrs		number of vgroup attributes	Vnattrs
_nmembers		number of vgroup members	Vntagrefs
_refnum		vgroup reference number	VQueryref
_tag		vgroup tag	VQuerytag
_version		vgroup version number	Vgetversion

V (Vgroup) API (pyhdf.V)¶

Introduction¶

Accessing the V module¶

Package components¶

Prerequisites¶

Summary of differences between the pyhdf and C V API¶

Error handling¶

V needs support from the HDF module¶

Classes summary¶

Attribute access: low and high level¶

Predefined attributes¶

Programming models¶

Creating and initializing a vgroup¶

Reading a vgroup¶

V (Vgroup) API (`pyhdf.V`)¶