Supported data types in PyTables¶
All PyTables datasets can handle the complete set of data types supported by the NumPy (see [NUMPY]) package in Python. The data types for table fields can be set via instances of the Col class and its descendants (see The Col class and its descendants), while the data type of array elements can be set through the use of the Atom class and its descendants (see The Atom class and its descendants).
PyTables uses ordinary strings to represent its types, with most of them matching the names of NumPy scalar types. Usually, a PyTables type consists of two parts: a kind and a precision in bits. The precision may be omitted in types with just one supported precision (like bool) or with a non-fixed size (like string).
There are eight kinds of types supported by PyTables:
- bool: Boolean (true/false) types. Supported precisions: 8 (default) bits.
- int: Signed integer types. Supported precisions: 8, 16, 32 (default) and 64 bits.
- uint: Unsigned integer types. Supported precisions: 8, 16, 32 (default) and 64 bits.
- float: Floating point types. Supported precisions: 16, 32, 64 (default) bits and extended precision floating point (see note on floating point types).
- complex: Complex number types. Supported precisions: 64 (32+32), 128 (64+64, default) bits and extended precision complex (see note on floating point types).
- string: Raw string types. Supported precisions: 8-bit positive multiples.
- time: Data/time types. Supported precisions: 32 and 64 (default) bits.
- enum: Enumerated types. Precision depends on base type.
Note
Floating point types.
The half precision floating point data type (float16) and extended precision ones (fload96, float128, complex192, complex256) are only available if numpy supports them on the host platform.
Also, in order to use the half precision floating point type (float16) it is required numpy >= 1.6.0.
The time and enum kinds area little bit special, since they represent HDF5 types which have no direct Python counterpart, though atoms of these kinds have a more-or-less equivalent NumPy data type.
There are two types of time: 4-byte signed integer (time32) and 8-byte double precision floating point (time64). Both of them reflect the number of seconds since the Unix epoch, i.e. Jan 1 00:00:00 UTC 1970. They are stored in memory as NumPy’s int32 and float64, respectively, and in the HDF5 file using the H5T_TIME class. Integer times are stored on disk as such, while floating point times are split into two signed integer values representing seconds and microseconds (beware: smaller decimals will be lost!).
PyTables also supports HDF5 H5T_ENUM enumerations (restricted sets of unique name and unique value pairs). The NumPy representation of an enumerated value (an Enum, see The Enum class) depends on the concrete base type used to store the enumeration in the HDF5 file. Currently, only scalar integer values (both signed and unsigned) are supported in enumerations. This restriction may be lifted when HDF5 supports other kinds on enumerated values.
Here you have a quick reference to the complete set of supported data types:
| Type Code | Description | C Type | Size (in bytes) | Python Counterpart |
|---|---|---|---|---|
| bool | boolean | unsigned char | 1 | bool |
| int8 | 8-bit integer | signed char | 1 | int |
| uint8 | 8-bit unsigned integer | unsigned char | 1 | int |
| int16 | 16-bit integer | short | 2 | int |
| uint16 | 16-bit unsigned integer | unsigned short | 2 | int |
| int32 | integer | int | 4 | int |
| uint32 | unsigned integer | unsigned int | 4 | long |
| int64 | 64-bit integer | long long | 8 | long |
| uint64 | unsigned 64-bit integer | unsigned long long | 8 | long |
| float16 [1] | half-precision float | 2 | ||
| float32 | single-precision float | float | 4 | float |
| float64 | double-precision float | double | 8 | float |
| float96 [1] [2] | extended precision float | 12 | ||
| float128 [1] [2] | extended precision float | 16 | ||
| complex64 | single-precision complex | struct {float r, i;} | 8 | complex |
| complex128 | double-precision complex | struct {double r, i;} | 16 | complex |
| complex192 [1] | extended precision complex | 24 | ||
| complex256 [1] | extended precision complex | 32 | ||
| string | arbitrary length string | char[] | str | |
| time32 | integer time | POSIX’s time_t | 4 | int |
| time64 | floating point time | POSIX’s struct timeval | 8 | float |
| enum | enumerated value | enum |
Footnotes
| [1] | (1, 2, 3, 4, 5) see the above note on floating point types. |
| [2] | (1, 2) currently in numpy. “float96” and “float128” are equivalent of “longdouble” i.e. 80 bit extended precision floating point. |
