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author | Matti Picus <matti.picus@gmail.com> | 2020-09-02 21:01:35 +0300 |
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committer | GitHub <noreply@github.com> | 2020-09-02 13:01:35 -0500 |
commit | 1f8ce6341159ebb0731c2c262f4576609210d2c8 (patch) | |
tree | aa10443358243366d776ad669c0cbcd383ed3634 /numpy/doc/structured_arrays.py | |
parent | e3c84a44b68966ab887a3623a0ff57169e508deb (diff) | |
download | numpy-1f8ce6341159ebb0731c2c262f4576609210d2c8.tar.gz |
MAINT, DOC: move informational files from numpy.doc.*.py to their *.rst counterparts (#17222)
* DOC: redistribute docstring-only content from numpy/doc
* DOC: post-transition clean-up
* DOC, MAINT: reskip doctests, fix a few easy ones
Diffstat (limited to 'numpy/doc/structured_arrays.py')
-rw-r--r-- | numpy/doc/structured_arrays.py | 646 |
1 files changed, 0 insertions, 646 deletions
diff --git a/numpy/doc/structured_arrays.py b/numpy/doc/structured_arrays.py deleted file mode 100644 index 359d4f7f4..000000000 --- a/numpy/doc/structured_arrays.py +++ /dev/null @@ -1,646 +0,0 @@ -""" -================= -Structured Arrays -================= - -Introduction -============ - -Structured arrays are ndarrays whose datatype is a composition of simpler -datatypes organized as a sequence of named :term:`fields <field>`. For example, -:: - - >>> x = np.array([('Rex', 9, 81.0), ('Fido', 3, 27.0)], - ... dtype=[('name', 'U10'), ('age', 'i4'), ('weight', 'f4')]) - >>> x - array([('Rex', 9, 81.), ('Fido', 3, 27.)], - dtype=[('name', 'U10'), ('age', '<i4'), ('weight', '<f4')]) - -Here ``x`` is a one-dimensional array of length two whose datatype is a -structure with three fields: 1. A string of length 10 or less named 'name', 2. -a 32-bit integer named 'age', and 3. a 32-bit float named 'weight'. - -If you index ``x`` at position 1 you get a structure:: - - >>> x[1] - ('Fido', 3, 27.0) - -You can access and modify individual fields of a structured array by indexing -with the field name:: - - >>> x['age'] - array([9, 3], dtype=int32) - >>> x['age'] = 5 - >>> x - array([('Rex', 5, 81.), ('Fido', 5, 27.)], - dtype=[('name', 'U10'), ('age', '<i4'), ('weight', '<f4')]) - -Structured datatypes are designed to be able to mimic 'structs' in the C -language, and share a similar memory layout. They are meant for interfacing with -C code and for low-level manipulation of structured buffers, for example for -interpreting binary blobs. For these purposes they support specialized features -such as subarrays, nested datatypes, and unions, and allow control over the -memory layout of the structure. - -Users looking to manipulate tabular data, such as stored in csv files, may find -other pydata projects more suitable, such as xarray, pandas, or DataArray. -These provide a high-level interface for tabular data analysis and are better -optimized for that use. For instance, the C-struct-like memory layout of -structured arrays in numpy can lead to poor cache behavior in comparison. - -.. _defining-structured-types: - -Structured Datatypes -==================== - -A structured datatype can be thought of as a sequence of bytes of a certain -length (the structure's :term:`itemsize`) which is interpreted as a collection -of fields. Each field has a name, a datatype, and a byte offset within the -structure. The datatype of a field may be any numpy datatype including other -structured datatypes, and it may also be a :term:`subarray data type` which -behaves like an ndarray of a specified shape. The offsets of the fields are -arbitrary, and fields may even overlap. These offsets are usually determined -automatically by numpy, but can also be specified. - -Structured Datatype Creation ----------------------------- - -Structured datatypes may be created using the function :func:`numpy.dtype`. -There are 4 alternative forms of specification which vary in flexibility and -conciseness. These are further documented in the -:ref:`Data Type Objects <arrays.dtypes.constructing>` reference page, and in -summary they are: - -1. A list of tuples, one tuple per field - - Each tuple has the form ``(fieldname, datatype, shape)`` where shape is - optional. ``fieldname`` is a string (or tuple if titles are used, see - :ref:`Field Titles <titles>` below), ``datatype`` may be any object - convertible to a datatype, and ``shape`` is a tuple of integers specifying - subarray shape. - - >>> np.dtype([('x', 'f4'), ('y', np.float32), ('z', 'f4', (2, 2))]) - dtype([('x', '<f4'), ('y', '<f4'), ('z', '<f4', (2, 2))]) - - If ``fieldname`` is the empty string ``''``, the field will be given a - default name of the form ``f#``, where ``#`` is the integer index of the - field, counting from 0 from the left:: - - >>> np.dtype([('x', 'f4'), ('', 'i4'), ('z', 'i8')]) - dtype([('x', '<f4'), ('f1', '<i4'), ('z', '<i8')]) - - The byte offsets of the fields within the structure and the total - structure itemsize are determined automatically. - -2. A string of comma-separated dtype specifications - - In this shorthand notation any of the :ref:`string dtype specifications - <arrays.dtypes.constructing>` may be used in a string and separated by - commas. The itemsize and byte offsets of the fields are determined - automatically, and the field names are given the default names ``f0``, - ``f1``, etc. :: - - >>> np.dtype('i8, f4, S3') - dtype([('f0', '<i8'), ('f1', '<f4'), ('f2', 'S3')]) - >>> np.dtype('3int8, float32, (2, 3)float64') - dtype([('f0', 'i1', (3,)), ('f1', '<f4'), ('f2', '<f8', (2, 3))]) - -3. A dictionary of field parameter arrays - - This is the most flexible form of specification since it allows control - over the byte-offsets of the fields and the itemsize of the structure. - - The dictionary has two required keys, 'names' and 'formats', and four - optional keys, 'offsets', 'itemsize', 'aligned' and 'titles'. The values - for 'names' and 'formats' should respectively be a list of field names and - a list of dtype specifications, of the same length. The optional 'offsets' - value should be a list of integer byte-offsets, one for each field within - the structure. If 'offsets' is not given the offsets are determined - automatically. The optional 'itemsize' value should be an integer - describing the total size in bytes of the dtype, which must be large - enough to contain all the fields. - :: - - >>> np.dtype({'names': ['col1', 'col2'], 'formats': ['i4', 'f4']}) - dtype([('col1', '<i4'), ('col2', '<f4')]) - >>> np.dtype({'names': ['col1', 'col2'], - ... 'formats': ['i4', 'f4'], - ... 'offsets': [0, 4], - ... 'itemsize': 12}) - dtype({'names':['col1','col2'], 'formats':['<i4','<f4'], 'offsets':[0,4], 'itemsize':12}) - - Offsets may be chosen such that the fields overlap, though this will mean - that assigning to one field may clobber any overlapping field's data. As - an exception, fields of :class:`numpy.object` type cannot overlap with - other fields, because of the risk of clobbering the internal object - pointer and then dereferencing it. - - The optional 'aligned' value can be set to ``True`` to make the automatic - offset computation use aligned offsets (see :ref:`offsets-and-alignment`), - as if the 'align' keyword argument of :func:`numpy.dtype` had been set to - True. - - The optional 'titles' value should be a list of titles of the same length - as 'names', see :ref:`Field Titles <titles>` below. - -4. A dictionary of field names - - The use of this form of specification is discouraged, but documented here - because older numpy code may use it. The keys of the dictionary are the - field names and the values are tuples specifying type and offset:: - - >>> np.dtype({'col1': ('i1', 0), 'col2': ('f4', 1)}) - dtype([('col1', 'i1'), ('col2', '<f4')]) - - This form is discouraged because Python dictionaries do not preserve order - in Python versions before Python 3.6, and the order of the fields in a - structured dtype has meaning. :ref:`Field Titles <titles>` may be - specified by using a 3-tuple, see below. - -Manipulating and Displaying Structured Datatypes ------------------------------------------------- - -The list of field names of a structured datatype can be found in the ``names`` -attribute of the dtype object:: - - >>> d = np.dtype([('x', 'i8'), ('y', 'f4')]) - >>> d.names - ('x', 'y') - -The field names may be modified by assigning to the ``names`` attribute using a -sequence of strings of the same length. - -The dtype object also has a dictionary-like attribute, ``fields``, whose keys -are the field names (and :ref:`Field Titles <titles>`, see below) and whose -values are tuples containing the dtype and byte offset of each field. :: - - >>> d.fields - mappingproxy({'x': (dtype('int64'), 0), 'y': (dtype('float32'), 8)}) - -Both the ``names`` and ``fields`` attributes will equal ``None`` for -unstructured arrays. The recommended way to test if a dtype is structured is -with `if dt.names is not None` rather than `if dt.names`, to account for dtypes -with 0 fields. - -The string representation of a structured datatype is shown in the "list of -tuples" form if possible, otherwise numpy falls back to using the more general -dictionary form. - -.. _offsets-and-alignment: - -Automatic Byte Offsets and Alignment ------------------------------------- - -Numpy uses one of two methods to automatically determine the field byte offsets -and the overall itemsize of a structured datatype, depending on whether -``align=True`` was specified as a keyword argument to :func:`numpy.dtype`. - -By default (``align=False``), numpy will pack the fields together such that -each field starts at the byte offset the previous field ended, and the fields -are contiguous in memory. :: - - >>> def print_offsets(d): - ... print("offsets:", [d.fields[name][1] for name in d.names]) - ... print("itemsize:", d.itemsize) - >>> print_offsets(np.dtype('u1, u1, i4, u1, i8, u2')) - offsets: [0, 1, 2, 6, 7, 15] - itemsize: 17 - -If ``align=True`` is set, numpy will pad the structure in the same way many C -compilers would pad a C-struct. Aligned structures can give a performance -improvement in some cases, at the cost of increased datatype size. Padding -bytes are inserted between fields such that each field's byte offset will be a -multiple of that field's alignment, which is usually equal to the field's size -in bytes for simple datatypes, see :c:member:`PyArray_Descr.alignment`. The -structure will also have trailing padding added so that its itemsize is a -multiple of the largest field's alignment. :: - - >>> print_offsets(np.dtype('u1, u1, i4, u1, i8, u2', align=True)) - offsets: [0, 1, 4, 8, 16, 24] - itemsize: 32 - -Note that although almost all modern C compilers pad in this way by default, -padding in C structs is C-implementation-dependent so this memory layout is not -guaranteed to exactly match that of a corresponding struct in a C program. Some -work may be needed, either on the numpy side or the C side, to obtain exact -correspondence. - -If offsets were specified using the optional ``offsets`` key in the -dictionary-based dtype specification, setting ``align=True`` will check that -each field's offset is a multiple of its size and that the itemsize is a -multiple of the largest field size, and raise an exception if not. - -If the offsets of the fields and itemsize of a structured array satisfy the -alignment conditions, the array will have the ``ALIGNED`` :attr:`flag -<numpy.ndarray.flags>` set. - -A convenience function :func:`numpy.lib.recfunctions.repack_fields` converts an -aligned dtype or array to a packed one and vice versa. It takes either a dtype -or structured ndarray as an argument, and returns a copy with fields re-packed, -with or without padding bytes. - -.. _titles: - -Field Titles ------------- - -In addition to field names, fields may also have an associated :term:`title`, -an alternate name, which is sometimes used as an additional description or -alias for the field. The title may be used to index an array, just like a -field name. - -To add titles when using the list-of-tuples form of dtype specification, the -field name may be specified as a tuple of two strings instead of a single -string, which will be the field's title and field name respectively. For -example:: - - >>> np.dtype([(('my title', 'name'), 'f4')]) - dtype([(('my title', 'name'), '<f4')]) - -When using the first form of dictionary-based specification, the titles may be -supplied as an extra ``'titles'`` key as described above. When using the second -(discouraged) dictionary-based specification, the title can be supplied by -providing a 3-element tuple ``(datatype, offset, title)`` instead of the usual -2-element tuple:: - - >>> np.dtype({'name': ('i4', 0, 'my title')}) - dtype([(('my title', 'name'), '<i4')]) - -The ``dtype.fields`` dictionary will contain titles as keys, if any -titles are used. This means effectively that a field with a title will be -represented twice in the fields dictionary. The tuple values for these fields -will also have a third element, the field title. Because of this, and because -the ``names`` attribute preserves the field order while the ``fields`` -attribute may not, it is recommended to iterate through the fields of a dtype -using the ``names`` attribute of the dtype, which will not list titles, as -in:: - - >>> for name in d.names: - ... print(d.fields[name][:2]) - (dtype('int64'), 0) - (dtype('float32'), 8) - -Union types ------------ - -Structured datatypes are implemented in numpy to have base type -:class:`numpy.void` by default, but it is possible to interpret other numpy -types as structured types using the ``(base_dtype, dtype)`` form of dtype -specification described in -:ref:`Data Type Objects <arrays.dtypes.constructing>`. Here, ``base_dtype`` is -the desired underlying dtype, and fields and flags will be copied from -``dtype``. This dtype is similar to a 'union' in C. - -Indexing and Assignment to Structured arrays -============================================ - -Assigning data to a Structured Array ------------------------------------- - -There are a number of ways to assign values to a structured array: Using python -tuples, using scalar values, or using other structured arrays. - -Assignment from Python Native Types (Tuples) -```````````````````````````````````````````` - -The simplest way to assign values to a structured array is using python tuples. -Each assigned value should be a tuple of length equal to the number of fields -in the array, and not a list or array as these will trigger numpy's -broadcasting rules. The tuple's elements are assigned to the successive fields -of the array, from left to right:: - - >>> x = np.array([(1, 2, 3), (4, 5, 6)], dtype='i8, f4, f8') - >>> x[1] = (7, 8, 9) - >>> x - array([(1, 2., 3.), (7, 8., 9.)], - dtype=[('f0', '<i8'), ('f1', '<f4'), ('f2', '<f8')]) - -Assignment from Scalars -``````````````````````` - -A scalar assigned to a structured element will be assigned to all fields. This -happens when a scalar is assigned to a structured array, or when an -unstructured array is assigned to a structured array:: - - >>> x = np.zeros(2, dtype='i8, f4, ?, S1') - >>> x[:] = 3 - >>> x - array([(3, 3., True, b'3'), (3, 3., True, b'3')], - dtype=[('f0', '<i8'), ('f1', '<f4'), ('f2', '?'), ('f3', 'S1')]) - >>> x[:] = np.arange(2) - >>> x - array([(0, 0., False, b'0'), (1, 1., True, b'1')], - dtype=[('f0', '<i8'), ('f1', '<f4'), ('f2', '?'), ('f3', 'S1')]) - -Structured arrays can also be assigned to unstructured arrays, but only if the -structured datatype has just a single field:: - - >>> twofield = np.zeros(2, dtype=[('A', 'i4'), ('B', 'i4')]) - >>> onefield = np.zeros(2, dtype=[('A', 'i4')]) - >>> nostruct = np.zeros(2, dtype='i4') - >>> nostruct[:] = twofield - Traceback (most recent call last): - ... - TypeError: Cannot cast array data from dtype([('A', '<i4'), ('B', '<i4')]) to dtype('int32') according to the rule 'unsafe' - -Assignment from other Structured Arrays -``````````````````````````````````````` - -Assignment between two structured arrays occurs as if the source elements had -been converted to tuples and then assigned to the destination elements. That -is, the first field of the source array is assigned to the first field of the -destination array, and the second field likewise, and so on, regardless of -field names. Structured arrays with a different number of fields cannot be -assigned to each other. Bytes of the destination structure which are not -included in any of the fields are unaffected. :: - - >>> a = np.zeros(3, dtype=[('a', 'i8'), ('b', 'f4'), ('c', 'S3')]) - >>> b = np.ones(3, dtype=[('x', 'f4'), ('y', 'S3'), ('z', 'O')]) - >>> b[:] = a - >>> b - array([(0., b'0.0', b''), (0., b'0.0', b''), (0., b'0.0', b'')], - dtype=[('x', '<f4'), ('y', 'S3'), ('z', 'O')]) - - -Assignment involving subarrays -`````````````````````````````` - -When assigning to fields which are subarrays, the assigned value will first be -broadcast to the shape of the subarray. - -Indexing Structured Arrays --------------------------- - -Accessing Individual Fields -``````````````````````````` - -Individual fields of a structured array may be accessed and modified by indexing -the array with the field name. :: - - >>> x = np.array([(1, 2), (3, 4)], dtype=[('foo', 'i8'), ('bar', 'f4')]) - >>> x['foo'] - array([1, 3]) - >>> x['foo'] = 10 - >>> x - array([(10, 2.), (10, 4.)], - dtype=[('foo', '<i8'), ('bar', '<f4')]) - -The resulting array is a view into the original array. It shares the same -memory locations and writing to the view will modify the original array. :: - - >>> y = x['bar'] - >>> y[:] = 11 - >>> x - array([(10, 11.), (10, 11.)], - dtype=[('foo', '<i8'), ('bar', '<f4')]) - -This view has the same dtype and itemsize as the indexed field, so it is -typically a non-structured array, except in the case of nested structures. - - >>> y.dtype, y.shape, y.strides - (dtype('float32'), (2,), (12,)) - -If the accessed field is a subarray, the dimensions of the subarray -are appended to the shape of the result:: - - >>> x = np.zeros((2, 2), dtype=[('a', np.int32), ('b', np.float64, (3, 3))]) - >>> x['a'].shape - (2, 2) - >>> x['b'].shape - (2, 2, 3, 3) - -Accessing Multiple Fields -``````````````````````````` - -One can index and assign to a structured array with a multi-field index, where -the index is a list of field names. - -.. warning:: - The behavior of multi-field indexes changed from Numpy 1.15 to Numpy 1.16. - -The result of indexing with a multi-field index is a view into the original -array, as follows:: - - >>> a = np.zeros(3, dtype=[('a', 'i4'), ('b', 'i4'), ('c', 'f4')]) - >>> a[['a', 'c']] - array([(0, 0.), (0, 0.), (0, 0.)], - dtype={'names':['a','c'], 'formats':['<i4','<f4'], 'offsets':[0,8], 'itemsize':12}) - -Assignment to the view modifies the original array. The view's fields will be -in the order they were indexed. Note that unlike for single-field indexing, the -dtype of the view has the same itemsize as the original array, and has fields -at the same offsets as in the original array, and unindexed fields are merely -missing. - -.. warning:: - In Numpy 1.15, indexing an array with a multi-field index returned a copy of - the result above, but with fields packed together in memory as if - passed through :func:`numpy.lib.recfunctions.repack_fields`. - - The new behavior as of Numpy 1.16 leads to extra "padding" bytes at the - location of unindexed fields compared to 1.15. You will need to update any - code which depends on the data having a "packed" layout. For instance code - such as:: - - >>> a[['a', 'c']].view('i8') # Fails in Numpy 1.16 - Traceback (most recent call last): - File "<stdin>", line 1, in <module> - ValueError: When changing to a smaller dtype, its size must be a divisor of the size of original dtype - - will need to be changed. This code has raised a ``FutureWarning`` since - Numpy 1.12, and similar code has raised ``FutureWarning`` since 1.7. - - In 1.16 a number of functions have been introduced in the - :mod:`numpy.lib.recfunctions` module to help users account for this - change. These are - :func:`numpy.lib.recfunctions.repack_fields`. - :func:`numpy.lib.recfunctions.structured_to_unstructured`, - :func:`numpy.lib.recfunctions.unstructured_to_structured`, - :func:`numpy.lib.recfunctions.apply_along_fields`, - :func:`numpy.lib.recfunctions.assign_fields_by_name`, and - :func:`numpy.lib.recfunctions.require_fields`. - - The function :func:`numpy.lib.recfunctions.repack_fields` can always be - used to reproduce the old behavior, as it will return a packed copy of the - structured array. The code above, for example, can be replaced with: - - >>> from numpy.lib.recfunctions import repack_fields - >>> repack_fields(a[['a', 'c']]).view('i8') # supported in 1.16 - array([0, 0, 0]) - - Furthermore, numpy now provides a new function - :func:`numpy.lib.recfunctions.structured_to_unstructured` which is a safer - and more efficient alternative for users who wish to convert structured - arrays to unstructured arrays, as the view above is often indeded to do. - This function allows safe conversion to an unstructured type taking into - account padding, often avoids a copy, and also casts the datatypes - as needed, unlike the view. Code such as: - - >>> b = np.zeros(3, dtype=[('x', 'f4'), ('y', 'f4'), ('z', 'f4')]) - >>> b[['x', 'z']].view('f4') - array([0., 0., 0., 0., 0., 0., 0., 0., 0.], dtype=float32) - - can be made safer by replacing with: - - >>> from numpy.lib.recfunctions import structured_to_unstructured - >>> structured_to_unstructured(b[['x', 'z']]) - array([0, 0, 0]) - - -Assignment to an array with a multi-field index modifies the original array:: - - >>> a[['a', 'c']] = (2, 3) - >>> a - array([(2, 0, 3.), (2, 0, 3.), (2, 0, 3.)], - dtype=[('a', '<i4'), ('b', '<i4'), ('c', '<f4')]) - -This obeys the structured array assignment rules described above. For example, -this means that one can swap the values of two fields using appropriate -multi-field indexes:: - - >>> a[['a', 'c']] = a[['c', 'a']] - -Indexing with an Integer to get a Structured Scalar -``````````````````````````````````````````````````` - -Indexing a single element of a structured array (with an integer index) returns -a structured scalar:: - - >>> x = np.array([(1, 2., 3.)], dtype='i, f, f') - >>> scalar = x[0] - >>> scalar - (1, 2., 3.) - >>> type(scalar) - <class 'numpy.void'> - -Unlike other numpy scalars, structured scalars are mutable and act like views -into the original array, such that modifying the scalar will modify the -original array. Structured scalars also support access and assignment by field -name:: - - >>> x = np.array([(1, 2), (3, 4)], dtype=[('foo', 'i8'), ('bar', 'f4')]) - >>> s = x[0] - >>> s['bar'] = 100 - >>> x - array([(1, 100.), (3, 4.)], - dtype=[('foo', '<i8'), ('bar', '<f4')]) - -Similarly to tuples, structured scalars can also be indexed with an integer:: - - >>> scalar = np.array([(1, 2., 3.)], dtype='i, f, f')[0] - >>> scalar[0] - 1 - >>> scalar[1] = 4 - -Thus, tuples might be thought of as the native Python equivalent to numpy's -structured types, much like native python integers are the equivalent to -numpy's integer types. Structured scalars may be converted to a tuple by -calling :func:`ndarray.item`:: - - >>> scalar.item(), type(scalar.item()) - ((1, 4.0, 3.0), <class 'tuple'>) - -Viewing Structured Arrays Containing Objects --------------------------------------------- - -In order to prevent clobbering object pointers in fields of -:class:`numpy.object` type, numpy currently does not allow views of structured -arrays containing objects. - -Structure Comparison --------------------- - -If the dtypes of two void structured arrays are equal, testing the equality of -the arrays will result in a boolean array with the dimensions of the original -arrays, with elements set to ``True`` where all fields of the corresponding -structures are equal. Structured dtypes are equal if the field names, -dtypes and titles are the same, ignoring endianness, and the fields are in -the same order:: - - >>> a = np.zeros(2, dtype=[('a', 'i4'), ('b', 'i4')]) - >>> b = np.ones(2, dtype=[('a', 'i4'), ('b', 'i4')]) - >>> a == b - array([False, False]) - -Currently, if the dtypes of two void structured arrays are not equivalent the -comparison fails, returning the scalar value ``False``. This behavior is -deprecated as of numpy 1.10 and will raise an error or perform elementwise -comparison in the future. - -The ``<`` and ``>`` operators always return ``False`` when comparing void -structured arrays, and arithmetic and bitwise operations are not supported. - -Record Arrays -============= - -As an optional convenience numpy provides an ndarray subclass, -:class:`numpy.recarray`, and associated helper functions in the -:mod:`numpy.rec` submodule, that allows access to fields of structured arrays -by attribute instead of only by index. Record arrays also use a special -datatype, :class:`numpy.record`, that allows field access by attribute on the -structured scalars obtained from the array. - -The simplest way to create a record array is with :func:`numpy.rec.array`:: - - >>> recordarr = np.rec.array([(1, 2., 'Hello'), (2, 3., "World")], - ... dtype=[('foo', 'i4'),('bar', 'f4'), ('baz', 'S10')]) - >>> recordarr.bar - array([ 2., 3.], dtype=float32) - >>> recordarr[1:2] - rec.array([(2, 3., b'World')], - dtype=[('foo', '<i4'), ('bar', '<f4'), ('baz', 'S10')]) - >>> recordarr[1:2].foo - array([2], dtype=int32) - >>> recordarr.foo[1:2] - array([2], dtype=int32) - >>> recordarr[1].baz - b'World' - -:func:`numpy.rec.array` can convert a wide variety of arguments into record -arrays, including structured arrays:: - - >>> arr = np.array([(1, 2., 'Hello'), (2, 3., "World")], - ... dtype=[('foo', 'i4'), ('bar', 'f4'), ('baz', 'S10')]) - >>> recordarr = np.rec.array(arr) - -The :mod:`numpy.rec` module provides a number of other convenience functions for -creating record arrays, see :ref:`record array creation routines -<routines.array-creation.rec>`. - -A record array representation of a structured array can be obtained using the -appropriate `view <numpy-ndarray-view>`_:: - - >>> arr = np.array([(1, 2., 'Hello'), (2, 3., "World")], - ... dtype=[('foo', 'i4'),('bar', 'f4'), ('baz', 'a10')]) - >>> recordarr = arr.view(dtype=np.dtype((np.record, arr.dtype)), - ... type=np.recarray) - -For convenience, viewing an ndarray as type :class:`np.recarray` will -automatically convert to :class:`np.record` datatype, so the dtype can be left -out of the view:: - - >>> recordarr = arr.view(np.recarray) - >>> recordarr.dtype - dtype((numpy.record, [('foo', '<i4'), ('bar', '<f4'), ('baz', 'S10')])) - -To get back to a plain ndarray both the dtype and type must be reset. The -following view does so, taking into account the unusual case that the -recordarr was not a structured type:: - - >>> arr2 = recordarr.view(recordarr.dtype.fields or recordarr.dtype, np.ndarray) - -Record array fields accessed by index or by attribute are returned as a record -array if the field has a structured type but as a plain ndarray otherwise. :: - - >>> recordarr = np.rec.array([('Hello', (1, 2)), ("World", (3, 4))], - ... dtype=[('foo', 'S6'),('bar', [('A', int), ('B', int)])]) - >>> type(recordarr.foo) - <class 'numpy.ndarray'> - >>> type(recordarr.bar) - <class 'numpy.recarray'> - -Note that if a field has the same name as an ndarray attribute, the ndarray -attribute takes precedence. Such fields will be inaccessible by attribute but -will still be accessible by index. - -""" |