DynamicTable How-To Guide

This is a user guide to interacting with DynamicTable objects.

Introduction

The DynamicTable class represents a column-based table to which you can add custom columns. It consists of a name, a description, a list of row IDs, and a list of columns. Columns are represented by objects of the class VectorData, including subclasses of VectorData, such as VectorIndex, and DynamicTableRegion.

Constructing a table

To create a DynamicTable, call the constructor for DynamicTable with a string name and string description. Specifying the arguments with keywords is recommended.

from hdmf.common import DynamicTable

table = DynamicTable(
    name='my_table',
    description='an example table',
)

Initializing columns

You can create a DynamicTable with particular columns by passing a list or tuple of VectorData objects for the columns argument in the constructor.

If the VectorData objects contain data values, then each VectorData object must contain the same number of rows as each other. A list of row IDs may be passed into the DynamicTable constructor using the id argument. If IDs are passed in, there should be the same number of rows as the column data. If IDs are not passed in, then the IDs will be set to range(len(column_data)) by default.

from hdmf.common import VectorData, VectorIndex

col1 = VectorData(
    name='col1',
    description='column #1',
    data=[1, 2],
)
col2 = VectorData(
    name='col2',
    description='column #2',
    data=['a', 'b'],
)

# this table will have two rows with ids 0 and 1
table = DynamicTable(
    name='my table',
    description='an example table',
    columns=[col1, col2],
)

# this table will have two rows with ids 0 and 1
table_set_ids = DynamicTable(
    name='my table',
    description='an example table',
    columns=[col1, col2],
    id=[0, 1],
)

If a list of integers in passed to id, DynamicTable automatically creates an ElementIdentifiers object, which is the data type that stores row IDs. The above command is equivalent to:

from hdmf.common.table import ElementIdentifiers

table_set_ids = DynamicTable(
    name='my table',
    description='an example table',
    columns=[col1, col2],
    id=ElementIdentifiers(name='id', data=[0, 1]),
)

Adding rows

You can also add rows to a DynamicTable using DynamicTable.add_row. A keyword argument for every column in the table must be supplied. You may also supply an optional row ID.

table.add_row(
    col1=3,
    col2='c',
    id=2,
)

Note

If no ID is supplied, the row ID is automatically set to the number of rows of the table prior to adding the new row. This can result in duplicate IDs. In general, IDs should be unique, but this is not enforced by default. Pass enforce_unique_id=True to DynamicTable.add_row to raise an error if the ID is set to an existing ID value.

# this row will have ID 3 by default
table.add_row(
    col1=4,
    col2='d',
)

Adding columns

You can add columns to a DynamicTable using DynamicTable.add_column. If the table already has rows, then the data argument must be supplied as a list of values, one for each row already in the table.

table.add_column(
    name='col3',
    description='column #3',
    data=[True, True, False, True],  # specify data for the 4 rows in the table
)

Enumerated (categorical) data

EnumData is a special type of column for storing an enumerated data type. This way each unique value is stored once, and the data references those values by index. Using this method is more efficient than storing a single value many times, and has the advantage of communicating to downstream tools that the data is categorical in nature.

from hdmf.common.table import EnumData

# this column has a length of 5, not 3. the first row has value "aa"
enum_col = EnumData(
    name='cell_type',
    description='this column holds categorical variables',
    data=[0, 1, 2, 1, 0],
    elements=['aa', 'bb', 'cc']
)

my_table = DynamicTable(
    name='my_table',
    description='an example table',
    columns=[enum_col],
)

/home/docs/checkouts/readthedocs.org/user_builds/hdmf/checkouts/677/src/hdmf/container.py:187: UserWarning: EnumData is experimental -- it may be removed in the future and is not guaranteed to maintain backward compatibility
  warn(_exp_warn_msg(cls))

Ragged array columns

A table column with a different number of elements for each row is called a “ragged array column”. To initialize a DynamicTable with a ragged array column, pass both the VectorIndex and its target VectorData in for the columns argument in the constructor. For instance, the following code creates a column called col1 where the first cell is [‘1a’, ‘1b’, ‘1c’] and the second cell is [‘2a’].

col1 = VectorData(
    name='col1',
    description='column #1',
    data=['1a', '1b', '1c', '2a'],
)
# the 3 signifies that elements 0 to 3 (exclusive) of the target column belong to the first row
# the 4 signifies that elements 3 to 4 (exclusive) of the target column belong to the second row
col1_ind = VectorIndex(
    name='col1_index',
    target=col1,
    data=[3, 4],
)

table_ragged_col = DynamicTable(
    name='my table',
    description='an example table',
    columns=[col1, col1_ind],
)

Note

By convention, the name of the VectorIndex should be the name of the target column with the added suffix “_index”.

VectorIndex.data provides the indices for how to break VectorData.data into cells

You can add an empty ragged array column to an existing DynamicTable by specifying index=True to DynamicTable.add_column. This method only works if run before any rows have been added to the table.

new_table = DynamicTable(
    name='my_table',
    description='an example table',
)

new_table.add_column(
    name='col4',
    description='column #4',
    index=True,
)

If the table already contains data, you must specify the new column values for the existing rows using the data argument and you must specify the end indices of the data argument that correspond to each row as a list/tuple/array of values for the index argument.

table.add_column(  # <-- this table already has 4 rows
    name='col4',
    description='column #4',
    data=[1, 0, -1, 0, -1, 1, 1, -1],
    index=[3, 4, 6, 8],  # specify the end indices (exclusive) of data for each row
)

Alternatively we may also define the ragged array data as a nested list and use the index argument to indicate the number of levels. In this case, the add_column function will automatically flatten the data array and compute the corresponding index vectors.

table.add_column(  # <-- this table already has 4 rows
    name='col5',
    description='column #5',
    data=[[[1, ], [2, 2]],       # row 1
          [[3, 3], ],            # row 2
          [[4, ], [5, 5]],       # row 3
          [[6, 6], [7, 7, 7]]],  # row 4
    index=2   # number of levels in the ragged array
)
# Show that the ragged array was converted to flat VectorData with a double VectorIndex
print("Flattened data: %s" % str(table.col5.data))
print("Level 1 index: %s" % str(table.col5_index.data))
print("Level 2 index: %s" % str(table.col5_index_index.data))

Flattened data: [1, 2, 2, 3, 3, 4, 5, 5, 6, 6, 7, 7, 7]
Level 1 index: [1, 3, 5, 6, 8, 10, 13]
Level 2 index: [2, 3, 5, 7]

Referencing rows of other tables

You can create a column that references rows of another table by adding a DynamicTableRegion object as a column of your DynamicTable. This is analogous to a foreign key in a relational database.

from hdmf.common.table import DynamicTableRegion

dtr_col = DynamicTableRegion(
    name='table1_ref',
    description='references rows of earlier table',
    data=[0, 1, 0, 0],  # refers to row indices of the 'table' variable
    table=table
)

data_col = VectorData(
    name='col2',
    description='column #2',
    data=['a', 'a', 'a', 'b'],
)

table2 = DynamicTable(
    name='my_table',
    description='an example table',
    columns=[dtr_col, data_col],
)

Here, the data of dtr_col maps to rows of table (0-indexed).

Note

The data values of DynamicTableRegion map to the row index, not the row ID, though if you are using default IDs, these values will be the same.

Reference more than one row of another table with a DynamicTableRegion indexed by a VectorIndex.

indexed_dtr_col = DynamicTableRegion(
    name='table1_ref2',
    description='references multiple rows of earlier table',
    data=[0, 0, 1, 1, 0, 0, 1],
    table=table
)

# row 0 refers to rows [0, 0], row 1 refers to rows [1], row 2 refers to rows [1, 0], row 3 refers to rows [0, 1] of
# the "table" variable
dtr_idx = VectorIndex(
    name='table1_ref2_index',
    target=indexed_dtr_col,
    data=[2, 3, 5, 7],
)

table3 = DynamicTable(
    name='my_table',
    description='an example table',
    columns=[dtr_idx, indexed_dtr_col],
)

Creating an expandable table

When using the default HDF5 backend, each column of these tables is an HDF5 Dataset, which by default are set in size. This means that once a file is written, it is not possible to add a new row. If you want to be able to save this file, load it, and add more rows to the table, you will need to set this up when you create the DynamicTable. You do this by wrapping the data with H5DataIO and the argument maxshape=(None, ).

from hdmf.backends.hdf5.h5_utils import H5DataIO

col1 = VectorData(
    name='expandable_col1',
    description='column #1',
    data=H5DataIO(data=[1, 2], maxshape=(None,)),
)
col2 = VectorData(
    name='expandable_col2',
    description='column #2',
    data=H5DataIO(data=['a', 'b'], maxshape=(None,)),
)

# don't forget to wrap the row IDs too!
ids = ElementIdentifiers(
    name='id',
    data=H5DataIO(data=[0, 1], maxshape=(None,)),
)

expandable_table = DynamicTable(
    name='expandable_table',
    description='an example table that can be expanded after being saved to a file',
    columns=[col1, col2],
    id=ids,
)

Now you can write the file, read it back, and run expandable_table.add_row(). In this example, we are setting maxshape to (None,), which means this is a 1-dimensional matrix that can expand indefinitely along its single dimension. You could also use an integer in place of None. For instance, maxshape=(8,) would allow the column to grow up to a length of 8. Whichever maxshape you choose, it should be the same for all VectorData and ElementIdentifiers objects in the DynamicTable, since they must always be the same length. The default ElementIdentifiers automatically generated when you pass a list of integers to the id argument of the DynamicTable constructor is not expandable, so do not forget to create a ElementIdentifiers object, and wrap that data as well. If any of the columns are indexed, the data argument of VectorIndex will also need to be wrapped with H5DataIO.

Converting the table to a pandas DataFrame

pandas is a popular data analysis tool, especially for working with tabular data. You can convert your DynamicTable to a DataFrame using DynamicTable.to_dataframe. Accessing the table as a DataFrame provides you with powerful, standard methods for indexing, selecting, and querying tabular data from pandas. This is the recommended method of reading data from your table. See also the pandas indexing documentation. Printing a DynamicTable as a DataFrame or displaying the DataFrame in Jupyter shows a more intuitive tabular representation of the data than printing the DynamicTable object.

df = table.to_dataframe()

Note

Changes to the DataFrame will not be saved in the DynamicTable.

Converting the table from a pandas DataFrame

If your data is already in a DataFrame, you can convert the DataFrame to a DynamicTable using the class method DynamicTable.from_dataframe.

table_from_df = DynamicTable.from_dataframe(
    name='my_table',
    df=df,
)

Accessing elements

To access an element in the i-th row in the column with name “col_name” in a DynamicTable, use square brackets notation: table[i, col_name]. You can also use a tuple of row index and column name within the square brackets.

table[0, 'col1']  # returns 1
table[(0, 'col1')]  # returns 1

1

If the column is a ragged array, instead of a single value being returned, a list of values for that element is returned.

table[0, 'col4']  # returns [1, 0, -1]

[1, 0, -1]

Standard Python and numpy slicing can be used for the row index.

import numpy as np

table[:2, 'col1']  # get a list of elements from the first two rows at column 'col1'
table[0:3:2, 'col1']  # get a list of elements from rows 0 to 3 (exclusive) in steps of 2 at column 'col1'
table[3::-1, 'col1']  # get a list of elements from rows 3 to 0 in reverse order at column 'col1'

# the following are equivalent to table[0:3:2, 'col1']
table[slice(0, 3, 2), 'col1']
table[np.s_[0:3:2], 'col1']
table[[0, 2], 'col1']
table[np.array([0, 2]), 'col1']

[1, 3]

If the column is a ragged array, instead of a list of row values being returned, a list of list elements for the selected rows is returned.

table[:2, 'col4']  # returns [[1, 0, -1], [0]]

[[1, 0, -1], [0]]

Note

You cannot supply a list/tuple for the column name. For this kind of access, first convert the DynamicTable to a DataFrame.

Accessing columns

To access all the values in a column, use square brackets with a colon for the row index: table[:, col_name]. If the column is a ragged array, a list of list elements is returned.

table[:, 'col1']  # returns [1, 2, 3, 4]
table[:, 'col4']  # returns [[1, 0, -1], [0], [-1, 1], [1, -1]]

[[1, 0, -1], [0], [-1, 1], [1, -1]]

Accessing rows

To access the i-th row in a DynamicTable, returned as a DataFrame, use the syntax table[i]. Standard Python and numpy slicing can be used for the row index.

table[0]  # get the 0th row of the table as a DataFrame
table[:2]  # get the first two rows
table[0:3:2]  # get rows 0 to 3 (exclusive) in steps of 2
table[3::-1]  # get rows 3 to 0 in reverse order

# the following are equivalent to table[0:3:2]
table[slice(0, 3, 2)]
table[np.s_[0:3:2]]
table[[0, 2]]
table[np.array([0, 2])]

	col1	col2	col3	col4	col5
id
0	1	a	True	[1, 0, -1]	[[1], [2, 2]]
2	3	c	False	[-1, 1]	[[4], [5, 5]]

Note

The syntax table[i] returns the i-th row, NOT the row with ID of i.

Iterating over rows

To iterate over the rows of a DynamicTable, first convert the DynamicTable to a DataFrame using DynamicTable.to_dataframe. For more information on iterating over a DataFrame, see https://pandas.pydata.org/pandas-docs/stable/user_guide/basics.html#iteration

df = table.to_dataframe()
for row in df.itertuples():
    print(row)

Pandas(Index=0, col1=1, col2='a', col3=True, col4=[1, 0, -1], col5=[[1], [2, 2]])
Pandas(Index=1, col1=2, col2='b', col3=True, col4=[0], col5=[[3, 3]])
Pandas(Index=2, col1=3, col2='c', col3=False, col4=[-1, 1], col5=[[4], [5, 5]])
Pandas(Index=3, col1=4, col2='d', col3=True, col4=[1, -1], col5=[[6, 6], [7, 7, 7]])

Accessing the column data types

To access the VectorData or VectorIndex object representing a column, you can use three different methods. Use the column name in square brackets, e.g., table[col_name], use the DynamicTable.get method, or use the column name as an attribute, e.g., table.col_name.

table['col1']
table.get('col1')  # equivalent to table['col1'] except this returns None if 'col1' is not found
table.get('col1', default=0)  # you can change the default return value
table.col1

<hdmf.common.table.VectorData object at 0x7f0da1fcb880>

Note

Using the column name as an attribute does NOT work if the column name is the same as a non-column name attribute or method of the DynamicTable class, e.g., name, description, object_id, parent, modified.

If the column is a ragged array, then the methods above will return the VectorIndex associated with the ragged array.

table['col4']
table.get('col4')  # equivalent to table['col4'] except this returns None if 'col4' is not found
table.get('col4', default=0)  # you can change the default return value

<hdmf.common.table.VectorIndex object at 0x7f0da80fe670>

Note

The attribute syntax table.col_name currently returns the VectorData instead of the VectorIndex for a ragged array. This is a known issue and will be fixed in a future version of HDMF.

Accessing elements from column data types

Standard Python and numpy slicing can be used on the VectorData or VectorIndex objects to access elements from column data. If the column is a ragged array, then instead of a list of row values being returned, a list of list elements for the selected rows is returned.

table['col1'][0]  # get the 0th element from column 'col1'
table['col1'][:2]  # get a list of the 0th and 1st elements
table['col1'][0:3:2]  # get a list of the 0th to 3rd (exclusive) elements in steps of 2
table['col1'][3::-1]  # get a list of the 3rd to 0th elements in reverse order

# the following are equivalent to table['col1'][0:3:2]
table['col1'][slice(0, 3, 2)]
table['col1'][np.s_[0:3:2]]
table['col1'][[0, 2]]
table['col1'][np.array([0, 2])]

# this slicing and indexing works for ragged array columns as well
table['col4'][:2]  # get a list of the 0th and 1st list elements

[[1, 0, -1], [0]]

Note

The syntax table[col_name][i] is equivalent to table[i, col_name].

Multi-dimensional columns

A column can be represented as a multi-dimensional rectangular array or a list of lists, each containing the same number of elements.

col5 = VectorData(
    name='col5',
    description='column #5',
    data=[['a', 'b', 'c'], ['d', 'e', 'f'], ['g', 'h', 'i']],
)

Ragged multi-dimensional columns

Each element within a column can be an n-dimensional array or list or lists. This is true for ragged array columns as well.

col6 = VectorData(
    name='col6',
    description='column #6',
    data=[['a', 'b', 'c'], ['d', 'e', 'f'], ['g', 'h', 'i']],
)
col6_ind = VectorIndex(
    name='col6_index',
    target=col6,
    data=[2, 3],
)

Nested ragged array columns

In the example above, the ragged array column above has two rows. The first row has two elements, where each element has 3 sub-elements. This can be thought of as a 2x3 array. The second row has one element with 3 sub-elements, or a 1x3 array. This works only if the data for col5 is a rectangular array, that is, each row element contains the same number of sub-elements. If each row element does not contain the same number of sub-elements, then a nested ragged array approach must be used instead.

A VectorIndex object can index another VectorIndex object. For example, the first row of a table might be a 2x3 array, the second row might be a 3x2 array, and the third row might be a 1x1 array. This cannot be represented by a singly indexed column, but can be represented by a nested ragged array column.

col7 = VectorData(
    name='col7',
    description='column #6',
    data=['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm'],
)
col7_ind = VectorIndex(
    name='col7_index',
    target=col7,
    data=[3, 6, 8, 10, 12, 13],
)
col7_ind_ind = VectorIndex(
    name='col7_index_index',
    target=col7_ind,
    data=[2, 5, 6],
)

# all indices must be added to the table
table_double_ragged_col = DynamicTable(
    name='my table',
    description='an example table',
    columns=[col7, col7_ind, col7_ind_ind],
)

Access the first row using the same syntax as before, except now a list of lists is returned. You can then index the resulting list of lists to access the individual elements.

table_double_ragged_col[0, 'col7']  # returns [['a', 'b', 'c'], ['d', 'e', 'f']]
table_double_ragged_col['col7'][0]  # same as line above

[['a', 'b', 'c'], ['d', 'e', 'f']]

Accessing the column named ‘col7’ using square bracket notation will return the top-level VectorIndex for the column. Accessing the column named ‘col7’ using dot notation will return the VectorData object

table_double_ragged_col['col7']  # returns col7_ind_ind
table_double_ragged_col.col7  # returns the col7 VectorData object

<hdmf.common.table.VectorData object at 0x7f0daaf23910>

Accessing data from a DynamicTable that contain references to rows of other DynamicTable objects

By default, when DynamicTable.__getitem__ and DynamicTable.get are supplied with an int, list of ints, numpy array, or a slice representing rows to return, a pandas DataFrame is returned. If the DynamicTable contains a DynamicTableRegion column that references rows of other DynamicTable objects, then by default, the DynamicTable.__getitem__ and DynamicTable.get methods will return row indices of the referenced table, and not the contents of the referenced table. To return the contents of the referenced table as a nested DataFrame containing only the referenced rows, use DynamicTable.get with index=False.

# create a new table of users
users_table = DynamicTable(
    name='users',
    description='a table containing data/metadata about users, one user per row',
)

# add simple columns to this table
users_table.add_column(
    name='first_name',
    description='the first name of the user',
)
users_table.add_column(
    name='last_name',
    description='the last name of the user',
)

# create a new table of addresses to reference
addresses_table = DynamicTable(
    name='addresses',
    description='a table containing data/metadata about addresses, one address per row',
)
addresses_table.add_column(
    name='street_address',
    description='the street number and address',
)
addresses_table.add_column(
    name='city',
    description='the city of the address',
)

# add rows to the addresses table
addresses_table.add_row(
    street_address='123 Main St',
    city='Springfield'
)
addresses_table.add_row(
    street_address='45 British Way',
    city='London'
)

# add a column to the users table that references rows of the addresses table
users_table.add_column(
    name='address',
    description='the address of the user',
    table=addresses_table
)

# add rows to the users table
users_table.add_row(
    first_name='Grace',
    last_name='Hopper',
    address=0  # <-- row index of the address table
)

users_table.add_row(
    first_name='Alan',
    last_name='Turing',
    address=1  # <-- row index of the address table
)

# get the first row of the users table
users_table.get(0)

	first_name	last_name	address
id
0	Grace	Hopper	0

# get the first row of the users table with a nested dataframe
users_table.get(0, index=False)

	first_name	last_name	address
id
0	Grace	Hopper	street_address city id ...

# get the first two rows of the users table
users_table.get([0, 1])

	first_name	last_name	address
id
0	Grace	Hopper	0
1	Alan	Turing	1

# get the first two rows of the users table with nested dataframes
# of the addresses table in the address column
users_table.get([0, 1], index=False)

	first_name	last_name	address
id
0	Grace	Hopper	street_address city id ...
1	Alan	Turing	street_address city id ...

Note

You can also get rows from a DynamicTable as a list of lists where the i-th nested list contains the values for the i-th row. This method is generally not recommended.

Displaying the contents of a table with references to another table

Earlier, we converted a DynamicTable to a DataFrame using DynamicTable.to_dataframe and printed the DataFrame to see its contents. This also works when the DynamicTable contains a column that references another table. However, the entries for this column for each row will be printed as a nested DataFrame. This can be difficult to read, so to view only the row indices of the referenced table, pass index=True to DynamicTable.to_dataframe.

users_df = users_table.to_dataframe(index=True)
users_df

	first_name	last_name	address
id
0	Grace	Hopper	0
1	Alan	Turing	1

You can then access the referenced table using the table attribute of the column object. This is useful when reading a table from a file where you may not have a variable to access the referenced table.

First, use DynamicTable.__getitem__ (square brackets notation) to get the DynamicTableRegion object representing the column. Then access its table attribute to get the addresses table and convert the table to a DataFrame.

address_column = users_table['address']
read_addresses_table = address_column.table
addresses_df = read_addresses_table.to_dataframe()

Get the addresses corresponding to the rows of the users table:

address_indices = users_df['address']  # pandas Series of row indices into the addresses table
addresses_df.iloc[address_indices]  # use .iloc because these are row indices not ID values

	street_address	city
id
0	123 Main St	Springfield
1	45 British Way	London

Note

The indices returned by users_df['address'] are row indices and not the ID values of the table. However, if you are using default IDs, these values will be the same.

Creating custom DynamicTable subclasses

TODO

Defining __columns__

TODO