# Data Storage and Access Using HDF5¶

An alternative data storage system is available for Larch, relying on the HDF5 format and the pytables package. This system is made available through a DT object, which wraps a tables.File object.

## Creating DT Objects¶

class larch.DT(filename, mode='a')

A wrapper for a pytables File used to get data for models.

This object wraps a tables.File, adding a number of methods designed specifically for working with choice-based data used in Larch.

Parameters: filename (str or None) – The filename of the HDF5/pytables to open. If None (the default) a named temporary file is created to serve as the backing for an in-memory HDF5 file, which is very fast as long as you’ve got enough memory to store the whole thing. mode (str) – The mode used to open the H5F file. Common values are ‘a’ for append and ‘r’ for read only. See pytables for more detail. complevel (int) – The compression level to use for new objects created. By default no compression is used, but substantial disk savings may be available by using it. inmemory (bool) – If True (defaults False), the H5FD_CORE driver is used and data will not in general be written to disk until the file is closed, when all accumulated changes will be written in a single batch. This can be fast if you have sufficent memory but if an error occurs all your intermediate changes can be lost. temp (bool) – If True (defaults False), the inmemory switch is activated and no changes will be written to disk when the file is closed. This is automatically set to true if the filename is None.
:param .. warning::: The normal constructor creates a DT object linked to an existing
HDF5 file. Editing the object edits the file as well.

Similar to the DB class, the DT class can be used with example data files.

static DT.Example(dataset='MTC', filename='{}.h5', temp=True)

Generate an example data object in memory.

Larch comes with a few example data sets, which are used in documentation and testing. This function copies the data into a HDF5 file, which you can freely edit without damaging the original data.

Parameters: dataset ({'MTC', 'SWISSMETRO', 'MINI', 'AIR'}) – Which example dataset should be used. filename (str) – A filename to open the HDF5 file (even in-memory files need a name). temp (bool) – The example database be created in-memory; if temp is false, the file will be dumped to disk when closed. An open connection to the HDF5 example data. DT
static DT.CSV_idco(filename, caseid=None, choice=None, weight=None, savename=None, alts={}, csv_args=(), csv_kwargs={}, complib='zlib', complevel=1, **kwargs)

Creates a new larch DT based on an idco Format CSV data file.

The input data file should be an idco Format data file, with the first line containing the column headings. The reader will attempt to determine the format (csv, tab-delimited, etc) automatically.

Parameters: Other Parameters: filename (str) – File name (absolute or relative) for CSV (or other text-based delimited) source data. caseid (str) – Column name that contains the unique case id’s. If the data is in idco format, case id’s can be generated automatically based on line numbers by setting caseid to None (the default). choice (str or None) – Column name that contains the id of the alternative that is selected (if applicable). If not given, and if choice is not included in alts below, no _choice_ h5f node will be autogenerated, and it will need to be set manually later. If the choices are given in alts (see below) then this value is ignored. weight (str or None) – Column name of the weight for each case. If None, defaults to equal weights. savename (str or None) – If not None, the name of the location to save the HDF5 file that is created. alts (dict) – A dictionary with keys of alt codes, and values of (alt name, avail column, choice column) tuples. The third item in the tuple can be omitted if choice is given. csv_args (tuple) – A tuple of positional arguments to pass to DT.import_idco() (and by extension to pandas.import_csv() or pandas.import_excel()). csv_kwargs (dict) – A dictionary of keyword arguments to pass to DT.import_idco() (and by extension to pandas.import_csv() or pandas.import_excel()). complib (str) – The compression library to use for the HDF5 file default filter. complevel (int) – The compression level to use for the HDF5 file default filter. Keyword arguments not listed here are passed to the :class:DT constructor. An open DT file. DT

## Importing Data¶

There are methods available to import data from external sources into the correct format for use with the larch DT facility.

DT.import_idco(filepath_or_buffer, caseid_column=None, *args, **kwargs)

Import an existing CSV or similar file in idco format into this HDF5 file.

This function relies on pandas.read_csv() to read and parse the input data. All arguments other than those described below are passed through to that function.

Parameters: filepath_or_buffer (str or buffer or pandas.DataFrame) – This argument will be fed directly to the pandas.read_csv() function. If a string is given and the file extension is ”.xlsx” then the pandas.read_excel() function will be used instead, ot if the file extension is ”.dbf” then simpledbf.Dbf5.to_dataframe() is used. Alternatively, you can just pass a pre-loaded pandas.DataFrame. caseid_column (None or str) – If given, this is the column of the input data file to use as caseids. It must be given if the caseids do not already exist in the HDF5 file. If it is given and the caseids do already exist, a LarchError is raised. LarchError – If caseids exist and are also given, or if the caseids are not integer values.
DT.import_idca(filepath_or_buffer, caseid_col, altid_col, choice_col=None, force_int_as_float=True, chunksize=inf)

Import an existing CSV or similar file in idca format into this HDF5 file.

This function relies on pandas.read_csv() to read and parse the input data. All arguments other than those described below are passed through to that function.

Parameters: filepath_or_buffer (str or buffer) – This argument will be fed directly to the pandas.read_csv() function. caseid_column (None or str) – If given, this is the column of the input data file to use as caseids. It must be given if the caseids do not already exist in the HDF5 file. If it is given and the caseids do already exist, a LarchError is raised. altid_col (None or str) – If given, this is the column of the input data file to use as altids. It must be given if the altids do not already exist in the HDF5 file. If it is given and the altids do already exist, a LarchError is raised. choice_col (None or str) – If given, use this column as the choice indicator. force_int_as_float (bool) – If True, data columns that appear to be integer values will still be stored as double precision floats (defaults to True). chunksize (int) – The number of rows of the source file to read as a chunk. Reading a giant file in moderate sized chunks can be much faster and less memory intensive than reading the entire file. LarchError – Various errors.

Notes

Chunking may not work on Mac OS X due to a known bug in the pandas.read_csv function.

## Required HDF5 Structure¶

To be used with Larch, the HDF5 file must have a particular structure. The group node structure is created automatically when you open a new DT object with a file that does not already have the necessary structure.

The details are as follows:

════════════════════════════════════════════════════════════════════════════════
larch.DT Validation for MTC.h5 (with mode 'w')
─────┬──────────────────────────────────────────────────────────────────────────
>>> │ There should be a designated larch group node under which all other
│ nodes reside.
─────┼──────────────────────────────────────────────────────────────────────────
│ CASES
>>> │ Under the top node, there must be an array node named caseids.
>>> │ The caseids array dtype should be Int64.
>>> │ The caseids array should be 1 dimensional.
├ Case Filtering ──────────────────────────────────────────────────────────
>>> │ If there may be some data cases that are not to be included in the
│ processing of the discrete choice model, there should be a node named
│ screen under the top node.
>>> │ If it exists screen must be a Bool array.
>>> │ And screen must be have the same shape as caseids.
─────┼──────────────────────────────────────────────────────────────────────────
│ ALTERNATIVES
>>> │ Under the top node, there should be a group named alts to hold
│ alternative data.
>>> │ Within the alts node, there should be an array node named altids to
│ hold the identifying code numbers of the alternatives.
>>> │ The altids array dtype should be Int64.
>>> │ The altids array should be one dimensional.
>>> │ Within the alts node, there should also be a VLArray node named names
│ to hold the names of the alternatives.
>>> │ The names node should hold unicode values.
>>> │ The altids and names arrays should be the same length, and this will
│ be the number of elemental alternatives represented in the data.
─────┼──────────────────────────────────────────────────────────────────────────
│ IDCO FORMAT DATA
>>> │ Under the top node, there should be a group named idco to hold that
│ data.
>>> │ Every child node name in idco must be a valid Python identifer (i.e.
│ starts with a letter or underscore, and only contains letters, numbers,
│ and underscores) and not a Python reserved keyword.
>>> │ Every child node in idco must be (1) an array node with shape the same
│ as caseids, or (2) a group node with child nodes _index_ as an array
│ with the correct shape and an integer dtype, and _values_ such that
│ _values_[_index_] reconstructs the desired data array.
├ Case Weights ────────────────────────────────────────────────────────────
>>> │ If the cases are to have non uniform weights, then there should a
│ _weight_ node (or a name link to a node) within the idco group.
>>> │ If weights are given, they should be of Float64 dtype.
─────┼──────────────────────────────────────────────────────────────────────────
│ IDCA FORMAT DATA
>>> │ Under the top node, there should be a group named idca to hold that
│ data.
>>> │ Every child node name in idca must be a valid Python identifer (i.e.
│ starts with a letter or underscore, and only contains letters, numbers,
│ and underscores) and not a Python reserved keyword.
>>> │ Every child node in idca must be (1) an array node with the first
│ dimension the same as the length of caseids, and the second dimension
│ the same as the length of altids, or (2) a group node with child nodes
│ _index_ as a 1-dimensional array with the same length as the length of
│ caseids and an integer dtype, and a 2-dimensional _values_ with the
│ second dimension the same as the length of altids, such that
│ _values_[_index_] reconstructs the desired data array.
├ Alternative Availability ────────────────────────────────────────────────
>>> │ If there may be some alternatives that are unavailable in some cases,
│ there should be a node named _avail_ under idca.
>>> │ If given as an array, it should contain an appropriately sized Bool array
│ indicating the availability status for each alternative.
>>> │ If given as a group, it should have an attribute named stack that is a
│ tuple of idco expressions indicating the availability status for each
│ alternative. The length and order of stack should match that of the
│ altid array.
├ Chosen Alternatives ────────────────────────────────────────────────────
>>> │ There should be a node named _choice_ under idca.
>>> │ If given as an array, it should be a Float64 array indicating the chosen-
│ ness for each alternative. Typically, this will take a value of 1.0 for
│ the alternative that is chosen and 0.0 otherwise, although it is possible
│ to have other values, including non-integer values, in some applications.
>>> │ If given as a group, it should have an attribute named stack that is a
│ tuple of idco expressions indicating the choice status for each
│ alternative. The length and order of stack should match that of the
│ altid array.
─────┼──────────────────────────────────────────────────────────────────────────
│ OTHER TECHNICAL DETAILS
>>> │ The set of child node names within idca and idco should not overlap
│ (i.e. there should be no node names that appear in both).
═════╧══════════════════════════════════════════════════════════════════════════


Note that the _choice_ and _avail_ nodes are special, they can be expressed as a stack if idco expressions instead of as a single idca array. To do so, replace the array node with a group node, and attach a stack attribute that gives the list of idco expressions. The list should match the list of alternatives. One way to do this automatically is to use the avail_idco and choice_idco attributes of the DT.

To check if your file has the correct structure, you can use the validate function:

DT.validate(log=<built-in function print>, errlog=None)

Generate a validation report for this DT.

The generated report is fairly detailed and describes each requirement for a valid DT file and whether or not it is met.

Parameters: log (callable) – Typically “print”, but can be replaced with a different callable to accept a series of unicode strings for each line in the report. errlog (callable or None) – By default, None. If not none, the report will print as with log but only if there are errors.
DT.choice_idco

The stack manager for choice data in idco format.

To set a stack of idco expressions to represent choice data, assign a dictionary to this attribute with keys as alternative codes and values as idco expressions.

You can also get and assign individual alternative values using the usual dictionary operations:

DT.choice_idco[key]            # get expression
DT.choice_idco[key] = value    # set expression

DT.avail_idco

The stack manager for avail data in idco format.

To set a stack of idco expressions to represent availability data, assign a dictionary to this attribute with keys as alternative codes and values as idco expressions.

You can also get and assign individual alternative values using the usual dictionary operations:

DT.avail_idco[key]            # get expression
DT.avail_idco[key] = value    # set expression


## Filtering Cases¶

It is common in discrete choice modeling to apply screening filters to a dataset before estimating parameters. These filters could be used for cleaning purposes (e.g. to remove apparently erroneous data) or to estimate a model for only a particular subset of observations (e.g. to pull out only home-based work trips from a data file that contains trips of many purposes).

DT.set_screen(exclude_idco=(), exclude_idca=(), exclude_unavail=False, exclude_unchoosable=False, dynamic=False)

Set a screen

DT.rescreen(exclude_idco=None, exclude_idca=None, exclude_unavail=None, exclude_unchoosable=None, dynamic=None)

Rebuild the screen based on the indicated exclusion criteria.

Parameters: exclude_idco (iterable of str) – A sequence of expressions that are evaluated as booleans using DT.array_idco(). For each case, if any of these expressions evaluates as true then the entire case is excluded. exclude_idca (iterable of (altcode,str)) – A sequence of (altcode, expression) tuples, where the expression is evaluated as boolean using DT.array_idca(). If the expression evaluates as true for any alternative matching any of the codes in the altcode part of the tuple (which can be an integer or an array of integers) then the case is excluded. Note that this excludes the whole case, not just the alternative in question. exclude_unavail (bool) – If true, then any case with no available alternatives is excluded. exclude_unchoosable (bool or int) – If true, then any case where an unavailable alternative is chosen is excluded. Set to an integer greater than 1 to increase the verbosity of the reporting.

Notes

Any method parameter can be omitted, in which case the previously used value of that parameter is retained. To explicitly clear previous screens, pass an empty tuple for each parameter.

DT.exclude_idco(expr, count=True)

Add an exclusion factor based on idco data.

This is primarily a convenience method, which calls rescreen. Future implementations may be modified to be more efficient.

Parameters: expr (str) – An expression to evaluate using array_idco(), with dtype set to bool. Any cases that evaluate as positive are excluded from the dataset when provisioning. count (bool) – Count the number of cases impacted by adding the screen. The number of cases excluded as a result of adding this exclusion factor. int
DT.exclude_idca(altids, expr, count=True)

Add an exclusion factor based on idca data.

This is primarily a convenience method, which calls rescreen. Future implementations may be modified to be more efficient.

Parameters: altids (iterable of int) – A set of alternative to consider. Any cases for which the expression evaluates as positive for any of the listed altids are excluded from the dataset when provisioning. expr (str) – An expression to evaluate using array_idca(), with dtype set to bool. count (bool) – Count the number of cases impacted by adding the screen.