.. include:: references.txt .. |join| replace:: :func:`~astropy.table.join` .. _table_operations: Table operations ***************** In this section we describe higher-level operations that can be used to generate a new table from one or more input tables. This includes: ======================= .. list-table:: :header-rows: 1 :widths: 28 52 20 * - Documentation - Description - Function * - `Grouped operations`_ - Group tables and columns by keys - `~astropy.table.Table.group_by` * - `Binning`_ - Binning tables - `~astropy.table.Table.group_by` * - `Stack vertically`_ - Concatenate input tables along rows - `~astropy.table.vstack` * - `Stack horizontally`_ - Concatenate input tables along columns - `~astropy.table.hstack` * - `Join`_ - Database-style join of two tables - `~astropy.table.join` * - `Unique rows`_ - Unique table rows by keys - `~astropy.table.unique` * - `Set difference`_ - Set difference of two tables - `~astropy.table.setdiff` * - `Table diff`_ - Generic difference of two simple tables - `~astropy.utils.diff.report_diff_values` .. _grouped-operations: Grouped operations ------------------ Sometimes in a table or table column there are natural groups within the dataset for which it makes sense to compute some derived values. A simple example is a list of objects with photometry from various observing runs:: >>> from astropy.table import Table >>> obs = Table.read("""name obs_date mag_b mag_v ... M31 2012-01-02 17.0 17.5 ... M31 2012-01-02 17.1 17.4 ... M101 2012-01-02 15.1 13.5 ... M82 2012-02-14 16.2 14.5 ... M31 2012-02-14 16.9 17.3 ... M82 2012-02-14 15.2 15.5 ... M101 2012-02-14 15.0 13.6 ... M82 2012-03-26 15.7 16.5 ... M101 2012-03-26 15.1 13.5 ... M101 2012-03-26 14.8 14.3 ... """, format='ascii') Table groups ^^^^^^^^^^^^^^ Now suppose we want the mean magnitudes for each object. We first group the data by the ``name`` column with the :func:`~astropy.table.Table.group_by` method. This returns a new table sorted by ``name`` which has a ``groups`` property specifying the unique values of ``name`` and the corresponding table rows:: >>> obs_by_name = obs.group_by('name') >>> print(obs_by_name) # doctest: +SKIP name obs_date mag_b mag_v ---- ---------- ----- ----- M101 2012-01-02 15.1 13.5 << First group (index=0, key='M101') M101 2012-02-14 15.0 13.6 M101 2012-03-26 15.1 13.5 M101 2012-03-26 14.8 14.3 M31 2012-01-02 17.0 17.5 << Second group (index=4, key='M31') M31 2012-01-02 17.1 17.4 M31 2012-02-14 16.9 17.3 M82 2012-02-14 16.2 14.5 << Third group (index=7, key='M83') M82 2012-02-14 15.2 15.5 M82 2012-03-26 15.7 16.5 << End of groups (index=10) >>> print(obs_by_name.groups.keys) name ---- M101 M31 M82 >>> print(obs_by_name.groups.indices) [ 0 4 7 10] The ``groups`` property is the portal to all grouped operations with tables and columns. It defines how the table is grouped via an array of the unique row key values and the indices of the group boundaries for those key values. The groups here correspond to the row slices ``0:4``, ``4:7``, and ``7:10`` in the ``obs_by_name`` table. The initial argument (``keys``) for the `~astropy.table.Table.group_by` function can take a number of input data types: - Single string value with a table column name (as shown above) - List of string values with table column names - Another |Table| or |Column| with same length as table - Numpy structured array with same length as table - Numpy homogeneous array with same length as table In all cases the corresponding row elements are considered as a tuple of values which form a key value that is used to sort the original table and generate the required groups. As an example, to get the average magnitudes for each object on each observing night, we would first group the table on both ``name`` and ``obs_date`` as follows:: >>> print(obs.group_by(['name', 'obs_date']).groups.keys) name obs_date ---- ---------- M101 2012-01-02 M101 2012-02-14 M101 2012-03-26 M31 2012-01-02 M31 2012-02-14 M82 2012-02-14 M82 2012-03-26 Manipulating groups ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Once you have applied grouping to a table then you can easily access the individual groups or subsets of groups. In all cases this returns a new grouped table. For instance to get the sub-table which corresponds to the second group (index=1) do:: >>> print(obs_by_name.groups[1]) name obs_date mag_b mag_v ---- ---------- ----- ----- M31 2012-01-02 17.0 17.5 M31 2012-01-02 17.1 17.4 M31 2012-02-14 16.9 17.3 To get the first and second groups together use a slice:: >>> groups01 = obs_by_name.groups[0:2] >>> print(groups01) name obs_date mag_b mag_v ---- ---------- ----- ----- M101 2012-01-02 15.1 13.5 M101 2012-02-14 15.0 13.6 M101 2012-03-26 15.1 13.5 M101 2012-03-26 14.8 14.3 M31 2012-01-02 17.0 17.5 M31 2012-01-02 17.1 17.4 M31 2012-02-14 16.9 17.3 >>> print(groups01.groups.keys) name ---- M101 M31 You can also supply a numpy array of indices or a boolean mask to select particular groups, e.g.:: >>> mask = obs_by_name.groups.keys['name'] == 'M101' >>> print(obs_by_name.groups[mask]) name obs_date mag_b mag_v ---- ---------- ----- ----- M101 2012-01-02 15.1 13.5 M101 2012-02-14 15.0 13.6 M101 2012-03-26 15.1 13.5 M101 2012-03-26 14.8 14.3 One can iterate over the group sub-tables and corresponding keys with:: >>> for key, group in zip(obs_by_name.groups.keys, obs_by_name.groups): ... print('****** {0} *******'.format(key['name'])) ... print(group) ... print('') ... ****** M101 ******* name obs_date mag_b mag_v ---- ---------- ----- ----- M101 2012-01-02 15.1 13.5 M101 2012-02-14 15.0 13.6 M101 2012-03-26 15.1 13.5 M101 2012-03-26 14.8 14.3 ****** M31 ******* name obs_date mag_b mag_v ---- ---------- ----- ----- M31 2012-01-02 17.0 17.5 M31 2012-01-02 17.1 17.4 M31 2012-02-14 16.9 17.3 ****** M82 ******* name obs_date mag_b mag_v ---- ---------- ----- ----- M82 2012-02-14 16.2 14.5 M82 2012-02-14 15.2 15.5 M82 2012-03-26 15.7 16.5 Column Groups ^^^^^^^^^^^^^^ Like |Table| objects, |Column| objects can also be grouped for subsequent manipulation with grouped operations. This can apply both to columns within a |Table| or bare |Column| objects. As for |Table|, the grouping is generated with the `~astropy.table.Table.group_by` method. The difference here is that there is no option of providing one or more column names since that doesn't make sense for a |Column|. Examples:: >>> from astropy.table import Column >>> import numpy as np >>> c = Column([1, 2, 3, 4, 5, 6], name='a') >>> key_vals = np.array(['foo', 'bar', 'foo', 'foo', 'qux', 'qux']) >>> cg = c.group_by(key_vals) >>> for key, group in zip(cg.groups.keys, cg.groups): ... print('****** {0} *******'.format(key)) ... print(group) ... print('') ... ****** bar ******* a --- 2 ****** foo ******* a --- 1 3 4 ****** qux ******* a --- 5 6 Aggregation ^^^^^^^^^^^^^^ Aggregation is the process of applying a specified reduction function to the values within each group for each non-key column. This function must accept a numpy array as the first argument and return a single scalar value. Common function examples are `numpy.sum`, `numpy.mean`, and `numpy.std`. For the example grouped table ``obs_by_name`` from above we compute the group means with the `~astropy.table.groups.TableGroups.aggregate` method:: >>> obs_mean = obs_by_name.groups.aggregate(np.mean) # doctest: +SKIP WARNING: Cannot aggregate column 'obs_date' [astropy.table.groups] >>> print(obs_mean) # doctest: +SKIP name mag_b mag_v ---- ----- ------ M101 15.0 13.725 M31 17.0 17.4 M82 15.7 15.5 It seems the magnitude values were successfully averaged, but what about the WARNING? Since the ``obs_date`` column is a string-type array, the `numpy.mean` function failed and raised an exception. Any time this happens then `~astropy.table.groups.TableGroups.aggregate` will issue a warning and then drop that column from the output result. Note that the ``name`` column is one of the ``keys`` used to determine the grouping so it is automatically ignored from aggregation. From a grouped table it is possible to select one or more columns on which to perform the aggregation:: >>> print(obs_by_name['mag_b'].groups.aggregate(np.mean)) # doctest: +SKIP mag_b ----- 15.0 17.0 15.1 >>> print(obs_by_name['name', 'mag_v', 'mag_b'].groups.aggregate(np.mean)) # doctest: +SKIP name mag_v mag_b ---- ------ ----- M101 13.725 15.0 M31 17.4 17.0 M82 15.5 15.7 .. above tests skipped as results look different in "not NUMPY_LT_1_14". A single column of data can be aggregated as well:: >>> c = Column([1, 2, 3, 4, 5, 6], name='a') >>> key_vals = np.array(['foo', 'bar', 'foo', 'foo', 'qux', 'qux']) >>> cg = c.group_by(key_vals) >>> cg_sums = cg.groups.aggregate(np.sum) >>> for key, cg_sum in zip(cg.groups.keys, cg_sums): ... print('Sum for {0} = {1}'.format(key, cg_sum)) ... Sum for bar = 2 Sum for foo = 8 Sum for qux = 11 If the specified function has a `numpy.ufunc.reduceat` method, this will be called instead. This can improve the performance by a factor of 10 to 100 (or more) for large unmasked tables or columns with many relatively small groups. It also allows for the use of certain numpy functions which normally take more than one input array but also work as reduction functions, like `numpy.add`. The numpy functions which should take advantage of using `numpy.ufunc.reduceat` include: `numpy.add`, `numpy.arctan2`, `numpy.bitwise_and`, `numpy.bitwise_or`, `numpy.bitwise_xor`, `numpy.copysign`, `numpy.divide`, `numpy.equal`, `numpy.floor_divide`, `numpy.fmax`, `numpy.fmin`, `numpy.fmod`, `numpy.greater_equal`, `numpy.greater`, `numpy.hypot`, `numpy.left_shift`, `numpy.less_equal`, `numpy.less`, `numpy.logaddexp2`, `numpy.logaddexp`, `numpy.logical_and`, `numpy.logical_or`, `numpy.logical_xor`, `numpy.maximum`, `numpy.minimum`, `numpy.mod`, `numpy.multiply`, `numpy.not_equal`, `numpy.power`, `numpy.remainder`, `numpy.right_shift`, `numpy.subtract` and `numpy.true_divide`. As special cases `numpy.sum` and `numpy.mean` are substituted with their respective reduceat methods. Filtering ^^^^^^^^^^ Table groups can be filtered by means of the `~astropy.table.groups.TableGroups.filter` method. This is done by supplying a function which is called for each group. The function which is passed to this method must accept two arguments: - ``table`` : |Table| object - ``key_colnames`` : list of columns in ``table`` used as keys for grouping It must then return either `True` or `False`. As an example, the following will select all table groups with only positive values in the non-key columns:: >>> def all_positive(table, key_colnames): ... colnames = [name for name in table.colnames if name not in key_colnames] ... for colname in colnames: ... if np.any(table[colname] < 0): ... return False ... return True An example of using this function is:: >>> t = Table.read(""" a b c ... -2 7.0 0 ... -2 5.0 1 ... 1 3.0 -5 ... 1 -2.0 -6 ... 1 1.0 7 ... 0 0.0 4 ... 3 3.0 5 ... 3 -2.0 6 ... 3 1.0 7""", format='ascii') >>> tg = t.group_by('a') >>> t_positive = tg.groups.filter(all_positive) >>> for group in t_positive.groups: ... print(group) ... print('') ... a b c --- --- --- -2 7.0 0 -2 5.0 1 a b c --- --- --- 0 0.0 4 As can be seen only the groups with ``a == -2`` and ``a == 0`` have all positive values in the non-key columns, so those are the ones that are selected. Likewise a grouped column can be filtered with the `~astropy.table.groups.ColumnGroups.filter`, method but in this case the filtering function takes only a single argument which is the column group. It still must return either `True` or `False`. For example:: def all_positive(column): if np.any(column < 0): return False return True .. _table_binning: Binning ------- A common tool in analysis is to bin a table based on some reference value. Examples: - Photometry of a binary star in several bands taken over a span of time which should be binned by orbital phase. - Reducing the sampling density for a table by combining 100 rows at a time. - Unevenly sampled historical data which should binned to four points per year. All of these examples of binning a table can be easily accomplished using `grouped operations`_. The examples in that section are focused on the case of discrete key values such as the name of a source. In this section we show a simple yet powerful way of applying grouped operations to accomplish binning on key values such as time, phase or row number. The common theme in all these cases is to convert the key value array into a new float- or int-valued array whose values are identical for rows in the same output bin. As an example, generate a fake light curve:: >>> year = np.linspace(2000.0, 2010.0, 200) # 200 observations over 10 years >>> period = 1.811 >>> y0 = 2005.2 >>> mag = 14.0 + 1.2 * np.sin(2 * np.pi * (year - y0) / period) >>> phase = ((year - y0) / period) % 1.0 >>> dat = Table([year, phase, mag], names=['year', 'phase', 'mag']) Now make an array that will be used for binning the data by 0.25 year intervals:: >>> year_bin = np.trunc(year / 0.25) This has the property that all samples in each 0.25 year bin have the same value of ``year_bin``. Think of ``year_bin`` as the bin number for ``year``. Then do the binning by grouping and immediately aggregating with ``np.mean``. >>> dat_grouped = dat.group_by(year_bin) >>> dat_binned = dat_grouped.groups.aggregate(np.mean) Then one might plot the results with ``plt.plot(dat_binned['year'], dat_binned['mag'], '.')``. Alternately one could bin into 10 phase bins:: >>> phase_bin = np.trunc(phase / 0.1) >>> dat_grouped = dat.group_by(phase_bin) >>> dat_binned = dat_grouped.groups.aggregate(np.mean) This time plot with ``plt.plot(dat_binned['phase'], dat_binned['mag'])``. .. _stack-vertically: Stack vertically -------------------- The |Table| class supports stacking tables vertically with the `~astropy.table.vstack` function. This process is also commonly known as concatenating or appending tables in the row direction. It corresponds roughly to the `numpy.vstack` function. For example, suppose one has two tables of observations with several column names in common:: >>> from astropy.table import Table, vstack >>> obs1 = Table.read("""name obs_date mag_b logLx ... M31 2012-01-02 17.0 42.5 ... M82 2012-10-29 16.2 43.5 ... M101 2012-10-31 15.1 44.5""", format='ascii') >>> obs2 = Table.read("""name obs_date logLx ... NGC3516 2011-11-11 42.1 ... M31 1999-01-05 43.1 ... M82 2012-10-30 45.0""", format='ascii') Now we can stack these two tables:: >>> print(vstack([obs1, obs2])) name obs_date mag_b logLx ------- ---------- ----- ----- M31 2012-01-02 17.0 42.5 M82 2012-10-29 16.2 43.5 M101 2012-10-31 15.1 44.5 NGC3516 2011-11-11 -- 42.1 M31 1999-01-05 -- 43.1 M82 2012-10-30 -- 45.0 Notice that the ``obs2`` table is missing the ``mag_b`` column, so in the stacked output table those values are marked as missing. This is the default behavior and corresponds to ``join_type='outer'``. There are two other allowed values for the ``join_type`` argument, ``'inner'`` and ``'exact'``:: >>> print(vstack([obs1, obs2], join_type='inner')) name obs_date logLx ------- ---------- ----- M31 2012-01-02 42.5 M82 2012-10-29 43.5 M101 2012-10-31 44.5 NGC3516 2011-11-11 42.1 M31 1999-01-05 43.1 M82 2012-10-30 45.0 >>> print(vstack([obs1, obs2], join_type='exact')) # doctest: +IGNORE_EXCEPTION_DETAIL Traceback (most recent call last): ... TableMergeError: Inconsistent columns in input arrays (use 'inner' or 'outer' join_type to allow non-matching columns) In the case of ``join_type='inner'``, only the common columns (the intersection) are present in the output table. When ``join_type='exact'`` is specified then `~astropy.table.vstack` requires that all the input tables have exactly the same column names. More than two tables can be stacked by supplying a list of table objects:: >>> obs3 = Table.read("""name obs_date mag_b logLx ... M45 2012-02-03 15.0 40.5""", format='ascii') >>> print(vstack([obs1, obs2, obs3])) name obs_date mag_b logLx ------- ---------- ----- ----- M31 2012-01-02 17.0 42.5 M82 2012-10-29 16.2 43.5 M101 2012-10-31 15.1 44.5 NGC3516 2011-11-11 -- 42.1 M31 1999-01-05 -- 43.1 M82 2012-10-30 -- 45.0 M45 2012-02-03 15.0 40.5 See also the sections on `Merging metadata`_ and `Merging column attributes`_ for details on how these characteristics of the input tables are merged in the single output table. Note also that you can use a single table row instead of a full table as one of the inputs. .. _stack-horizontally: Stack horizontally --------------------- The |Table| class supports stacking tables horizontally (in the column-wise direction) with the `~astropy.table.hstack` function. It corresponds roughly to the `numpy.hstack` function. For example, suppose one has the following two tables:: >>> from astropy.table import Table, hstack >>> t1 = Table.read("""a b c ... 1 foo 1.4 ... 2 bar 2.1 ... 3 baz 2.8""", format='ascii') >>> t2 = Table.read("""d e ... ham eggs ... spam toast""", format='ascii') Now we can stack these two tables horizontally:: >>> print(hstack([t1, t2])) a b c d e --- --- --- ---- ----- 1 foo 1.4 ham eggs 2 bar 2.1 spam toast 3 baz 2.8 -- -- As with `~astropy.table.vstack`, there is an optional ``join_type`` argument that can take values ``'inner'``, ``'exact'``, and ``'outer'``. The default is ``'outer'``, which effectively takes the union of available rows and masks out any missing values. This is illustrated in the example above. The other options give the intersection of rows, where ``'exact'`` requires that all tables have exactly the same number of rows:: >>> print(hstack([t1, t2], join_type='inner')) a b c d e --- --- --- ---- ----- 1 foo 1.4 ham eggs 2 bar 2.1 spam toast >>> print(hstack([t1, t2], join_type='exact')) # doctest: +IGNORE_EXCEPTION_DETAIL Traceback (most recent call last): ... TableMergeError: Inconsistent number of rows in input arrays (use 'inner' or 'outer' join_type to allow non-matching rows) More than two tables can be stacked by supplying a list of table objects. The example below also illustrates the behavior when there is a conflict in the input column names (see the section on `Column renaming`_ for details):: >>> t3 = Table.read("""a b ... M45 2012-02-03""", format='ascii') >>> print(hstack([t1, t2, t3])) a_1 b_1 c d e a_3 b_3 --- --- --- ---- ----- --- ---------- 1 foo 1.4 ham eggs M45 2012-02-03 2 bar 2.1 spam toast -- -- 3 baz 2.8 -- -- -- -- The metadata from the input tables is merged by the process described in the `Merging metadata`_ section. Note also that you can use a single table row instead of a full table as one of the inputs. .. _table-join: Join -------------- The |Table| class supports the `database join `_ operation. This provides a flexible and powerful way to combine tables based on the values in one or more key columns. For example, suppose one has two tables of observations, the first with B and V magnitudes and the second with X-ray luminosities of an overlapping (but not identical) sample:: >>> from astropy.table import Table, join >>> optical = Table.read("""name obs_date mag_b mag_v ... M31 2012-01-02 17.0 16.0 ... M82 2012-10-29 16.2 15.2 ... M101 2012-10-31 15.1 15.5""", format='ascii') >>> xray = Table.read(""" name obs_date logLx ... NGC3516 2011-11-11 42.1 ... M31 1999-01-05 43.1 ... M82 2012-10-29 45.0""", format='ascii') The |join| method allows one to merge these two tables into a single table based on matching values in the "key columns". By default the key columns are the set of columns that are common to both tables. In this case the key columns are ``name`` and ``obs_date``. We can find all the observations of the same object on the same date as follows:: >>> opt_xray = join(optical, xray) >>> print(opt_xray) name obs_date mag_b mag_v logLx ---- ---------- ----- ----- ----- M82 2012-10-29 16.2 15.2 45.0 We can perform the match only by ``name`` by providing the ``keys`` argument, which can be either a single column name or a list of column names:: >>> print(join(optical, xray, keys='name')) name obs_date_1 mag_b mag_v obs_date_2 logLx ---- ---------- ----- ----- ---------- ----- M31 2012-01-02 17.0 16.0 1999-01-05 43.1 M82 2012-10-29 16.2 15.2 2012-10-29 45.0 This output table has all observations that have both optical and X-ray data for an object (M31 and M82). Notice that since the ``obs_date`` column occurs in both tables it has been split into two columns, ``obs_date_1`` and ``obs_date_2``. The values are taken from the "left" (``optical``) and "right" (``xray``) tables, respectively. Different join options ^^^^^^^^^^^^^^^^^^^^^^ The table joins so far are known as "inner" joins and represent the strict intersection of the two tables on the key columns. If one wants to make a new table which has *every* row from the left table and includes matching values from the right table when available, this is known as a left join:: >>> print(join(optical, xray, join_type='left')) name obs_date mag_b mag_v logLx ---- ---------- ----- ----- ----- M101 2012-10-31 15.1 15.5 -- M31 2012-01-02 17.0 16.0 -- M82 2012-10-29 16.2 15.2 45.0 Two of the observations do not have X-ray data, as indicated by the ``--`` in the table. When there are any missing values the output will be a masked table (see :ref:`masking_and_missing_values` for more information). You might be surprised that there is no X-ray data for M31 in the output. Remember that the default matching key includes both ``name`` and ``obs_date``. Specifying the key as only the ``name`` column gives:: >>> print(join(optical, xray, join_type='left', keys='name')) name obs_date_1 mag_b mag_v obs_date_2 logLx ---- ---------- ----- ----- ---------- ----- M101 2012-10-31 15.1 15.5 -- -- M31 2012-01-02 17.0 16.0 1999-01-05 43.1 M82 2012-10-29 16.2 15.2 2012-10-29 45.0 Likewise one can construct a new table with every row of the right table and matching left values (when available) using ``join_type='right'``. Finally, to make a table with the union of rows from both tables do an "outer" join:: >>> print(join(optical, xray, join_type='outer')) name obs_date mag_b mag_v logLx ------- ---------- ----- ----- ----- M101 2012-10-31 15.1 15.5 -- M31 1999-01-05 -- -- 43.1 M31 2012-01-02 17.0 16.0 -- M82 2012-10-29 16.2 15.2 45.0 NGC3516 2011-11-11 -- -- 42.1 In all cases the output join table will be sorted by the key column(s) and in general will not preserve the row order of the input tables. Non-identical key column names ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ The |join| function requires the key column names to be identical in the two tables. However, in the following one table has a ``'name'`` column while the other has an ``'obj_id'`` column:: >>> optical = Table.read("""name obs_date mag_b mag_v ... M31 2012-01-02 17.0 16.0 ... M82 2012-10-29 16.2 15.2 ... M101 2012-10-31 15.1 15.5""", format='ascii') >>> xray_1 = Table.read(""" obj_id obs_date logLx ... NGC3516 2011-11-11 42.1 ... M31 1999-01-05 43.1 ... M82 2012-10-29 45.0""", format='ascii') In order to perform a match based on the names of the objects, one has to temporarily rename one of the columns mentioned above, right before creating the new table:: >>> xray_1.rename_column('obj_id', 'name') >>> opt_xray_1 = join(optical, xray_1, keys='name') >>> xray_1.rename_column('name', 'obj_id') >>> print(opt_xray_1) name obs_date_1 mag_b mag_v obs_date_2 logLx ---- ---------- ----- ----- ---------- ----- M31 2012-01-02 17.0 16.0 1999-01-05 43.1 M82 2012-10-29 16.2 15.2 2012-10-29 45.0 The original ``xray_1`` table remains unchanged after the operation:: >>> print(xray_1) obj_id obs_date logLx ------- ---------- ----- NGC3516 2011-11-11 42.1 M31 1999-01-05 43.1 M82 2012-10-29 45.0 Identical key values ^^^^^^^^^^^^^^^^^^^^ The |Table| join operation works even if there are multiple rows with identical key values. For example the following tables have multiple rows for the key column ``x``:: >>> from astropy.table import Table, join >>> left = Table([[0, 1, 1, 2], ['L1', 'L2', 'L3', 'L4']], names=('key', 'L')) >>> right = Table([[1, 1, 2, 4], ['R1', 'R2', 'R3', 'R4']], names=('key', 'R')) >>> print(left) key L --- --- 0 L1 1 L2 1 L3 2 L4 >>> print(right) key R --- --- 1 R1 1 R2 2 R3 4 R4 Doing an outer join on these tables shows that what is really happening is a `Cartesian product `_. For each matching key, every combination of the left and right tables is represented. When there is no match in either the left or right table, the corresponding column values are designated as missing. .. doctest-skip:: win32 >>> print(join(left, right, join_type='outer')) key L R --- --- --- 0 L1 -- 1 L2 R1 1 L2 R2 1 L3 R1 1 L3 R2 2 L4 R3 4 -- R4 .. note:: The output table is sorted on the key columns, but when there are rows with identical keys the output order in the non-key columns is not guaranteed to be identical across installations. In the example above the order within the four rows with ``key == 1`` can vary. An inner join is the same but only returns rows where there is a key match in both the left and right tables: .. doctest-skip:: win32 >>> print(join(left, right, join_type='inner')) key L R --- --- --- 1 L2 R1 1 L2 R2 1 L3 R1 1 L3 R2 2 L4 R3 Conflicts in the input table names are handled by the process described in the section on `Column renaming`_. See also the sections on `Merging metadata`_ and `Merging column attributes`_ for details on how these characteristics of the input tables are merged in the single output table. Merging details -------------------- When combining two or more tables there is the need to merge certain characteristics in the inputs and potentially resolve conflicts. This section describes the process. Column renaming ^^^^^^^^^^^^^^^^^ In cases where the input tables have conflicting column names, there is a mechanism to generate unique output column names. There are two keyword arguments that control the renaming behavior: ``table_names`` Two-element list of strings that provide a name for the tables being joined. By default this is ``['1', '2', ...]``, where the numbers correspond to the input tables. ``uniq_col_name`` String format specifier with a default value of ``'{col_name}_{table_name}'``. This is most easily understood by example using the ``optical`` and ``xray`` tables in the |join| example defined previously:: >>> print(join(optical, xray, keys='name', ... table_names=['OPTICAL', 'XRAY'], ... uniq_col_name='{table_name}_{col_name}')) name OPTICAL_obs_date mag_b mag_v XRAY_obs_date logLx ---- ---------------- ----- ----- ------------- ----- M31 2012-01-02 17.0 16.0 1999-01-05 43.1 M82 2012-10-29 16.2 15.2 2012-10-29 45.0 Merging metadata ^^^^^^^^^^^^^^^^^^^ |Table| objects can have associated metadata: - ``Table.meta``: table-level metadata as an ordered dictionary - ``Column.meta``: per-column metadata as an ordered dictionary The table operations described here handle the task of merging the metadata in the input tables into a single output structure. Because the metadata can be arbitrarily complex there is no unique way to do the merge. The current implementation uses a simple recursive algorithm with four rules: - `dict` elements are merged by keys - Conflicting `list` or `tuple` elements are concatenated - Conflicting `dict` elements are merged by recursively calling the merge function - Conflicting elements that are not both `list`, `tuple`, or `dict` will follow the following rules: - If both metadata values are identical, the output is set to this value - If one of the conflicting metadata values is `None`, the other value is picked - If both metadata values are different and neither is `None`, the one for the last table in the list is picked By default, a warning is emitted in the last case (both metadata values are not `None`). The warning can be silenced or made into an exception using the ``metadata_conflicts`` argument to :func:`~astropy.table.hstack`, :func:`~astropy.table.vstack`, or :func:`~astropy.table.join`. The ``metadata_conflicts`` option can be set to: - ``'silent'`` - no warning is emitted, the value for the last table is silently picked - ``'warn'`` - a warning is emitted, the value for the last table is picked - ``'error'`` - an exception is raised The default strategies for merging metadata can be augmented or customized by defining subclasses of the `~astropy.utils.metadata.MergeStrategy` base class. In most cases one also will use the `~astropy.utils.metadata.enable_merge_strategies` for enable the custom strategies. The linked documentation strings provide details. Merging column attributes ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ In addition to the table and column ``meta`` attributes, the column attributes ``unit``, ``format``, and ``description`` are merged by going through the input tables in order and taking the first value which is defined (i.e. is not None). For example:: >>> from astropy.table import Column, Table, vstack >>> col1 = Column([1], name='a') >>> col2 = Column([2], name='a', unit='cm') >>> col3 = Column([3], name='a', unit='m') >>> t1 = Table([col1]) >>> t2 = Table([col2]) >>> t3 = Table([col3]) >>> out = vstack([t1, t2, t3]) # doctest: +SKIP WARNING: MergeConflictWarning: In merged column 'a' the 'unit' attribute does not match (cm != m). Using m for merged output [astropy.table.operations] >>> out['a'].unit # doctest: +SKIP Unit("m") The rules for merging are as for `Merging metadata`_, and the ``metadata_conflicts`` option also controls the merging of column attributes. .. _unique-rows: Unique rows ----------- Sometimes it makes sense to use only rows with unique key columns or even fully unique rows from a table. This can be done using the above described :func:`~astropy.table.Table.group_by` method and ``groups`` attribute, or with the `~astropy.table.unique` convenience function. The `~astropy.table.unique` function returns with a sorted table containing the first row for each unique ``keys`` column value. If no ``keys`` is provided it returns with a sorted table containing all the fully unique rows. A simple example is a list of objects with photometry from various observing runs. Using ``'name'`` as the only ``keys``, it returns with the first occurrence of each of the three targets:: >>> from astropy import table >>> obs = table.Table.read("""name obs_date mag_b mag_v ... M31 2012-01-02 17.0 17.5 ... M82 2012-02-14 16.2 14.5 ... M101 2012-01-02 15.1 13.5 ... M31 2012-01-02 17.1 17.4 ... M101 2012-01-02 15.1 13.5 ... M82 2012-02-14 16.2 14.5 ... M31 2012-02-14 16.9 17.3 ... M82 2012-02-14 15.2 15.5 ... M101 2012-02-14 15.0 13.6 ... M82 2012-03-26 15.7 16.5 ... M101 2012-03-26 15.1 13.5 ... M101 2012-03-26 14.8 14.3 ... """, format='ascii') >>> unique_by_name = table.unique(obs, keys='name') >>> print(unique_by_name) name obs_date mag_b mag_v ---- ---------- ----- ----- M101 2012-01-02 15.1 13.5 M31 2012-01-02 17.0 17.5 M82 2012-02-14 16.2 14.5 Using multiple columns as ``keys``:: >>> unique_by_name_date = table.unique(obs, keys=['name', 'obs_date']) >>> print(unique_by_name_date) name obs_date mag_b mag_v ---- ---------- ----- ----- M101 2012-01-02 15.1 13.5 M101 2012-02-14 15.0 13.6 M101 2012-03-26 15.1 13.5 M31 2012-01-02 17.0 17.5 M31 2012-02-14 16.9 17.3 M82 2012-02-14 16.2 14.5 M82 2012-03-26 15.7 16.5 .. _set-difference: Set difference -------------- A set difference will tell you the elements that are contained in one set but not in the other. This concept can be applied to rows of a table by using the `~astropy.table.setdiff` function. You provide the function with two input tables and it will return all rows in the first table which do not occur in the second table. The optional ``keys`` parameter specifies the names of columns that are used to match table rows. This can be a subset of the full list of columns, but both the first and second tables must contain all columns specified by ``keys``. If not provided then ``keys`` defaults to all column names in the first table. If no different rows are found the `~astropy.table.setdiff` function will return an empty table. The example below illustrates finding the set difference of two observation lists using a common subset of the columns in two tables.:: >>> from astropy.table import Table, setdiff >>> cat_1 = Table.read("""name obs_date mag_b mag_v ... M31 2012-01-02 17.0 16.0 ... M82 2012-10-29 16.2 15.2 ... M101 2012-10-31 15.1 15.5""", format='ascii') >>> cat_2 = Table.read(""" name obs_date logLx ... NGC3516 2011-11-11 42.1 ... M31 2012-01-02 43.1 ... M82 2012-10-29 45.0""", format='ascii') >>> sdiff = setdiff(cat_1, cat_2, keys=['name', 'obs_date']) >>> print(sdiff) name obs_date mag_b mag_v ---- ---------- ----- ----- M101 2012-10-31 15.1 15.5 In this example there is a column in the first table that is not present in the second table, so the ``keys`` parameter must be used to specify the desired column names. .. _table-diff: Table diff ---------- To compare two simple tables, you can use :func:`~astropy.utils.diff.report_diff_values`, which would produce a report identical to :ref:`FITS diff `. The example below illustrates finding the difference between two simple tables:: >>> from astropy.table import Table >>> from astropy.utils.diff import report_diff_values >>> import sys >>> cat_1 = Table.read("""name obs_date mag_b mag_v ... M31 2012-01-02 17.0 16.0 ... M82 2012-10-29 16.2 15.2 ... M101 2012-10-31 15.1 15.5""", format='ascii') >>> cat_2 = Table.read("""name obs_date mag_b mag_v ... M31 2012-01-02 17.0 16.5 ... M82 2012-10-29 16.2 15.2 ... M101 2012-10-30 15.1 15.5 ... NEW 2018-05-08 nan 9.0""", format='ascii') >>> identical = report_diff_values(cat_1, cat_2, fileobj=sys.stdout) name obs_date mag_b mag_v ---- ---------- ----- ----- a> M31 2012-01-02 17.0 16.0 ? ^ b> M31 2012-01-02 17.0 16.5 ? ^ M82 2012-10-29 16.2 15.2 a> M101 2012-10-31 15.1 15.5 ? ^ b> M101 2012-10-30 15.1 15.5 ? ^ b> NEW 2018-05-08 nan 9.0 >>> identical False