Typesystem

This page describes the data types introduced in pSeven and type conversion rules.

Introduction

pSeven typesystem distinguishes between Boolean, integer, real (floating point) and string values. These values can be scalar, vector, or matrix. As a result of taking all possible combinations, there are 12 base types: BoolScalar, IntScalar, RealScalar, StringScalar and so on for vectors and matrices (for example, RealVector, StringMatrix).

Special types are File (file object), Blob (arbitrary binary data), List, and Dict (pSeven implementation of heterogeneous list and associative array types). List and Dict are heterogeneous containers — they allow mixing elements of different types and can hold elements of any base or special type.

Lastly, there is the Undefined type (which actually is a special value, like None in Python). Undefined cannot be an element of a List or a Dict.

In total, there are 17 typenames (12 base, 4 special, and Undefined). Detailed information on individual types (including their standard string representations used here) can be found in the pSeven Data Types reference. This page describes pSeven typesystem concepts and type conversion rules in general.

Base Types

Base pSeven data types can represent:

Boolean values are either True or False, other representations are not supported.

Integer values are 32-bit signed integers, floating point numbers are double precision floats, represented by numeric literals.

String literals are always enclosed in double quotes.

Vectors and matrices are homogeneous: all elements are of the same type. All indices are zero-based. Matrices are indexed in row major order. String form also follows this order: for example, in the ((11, 12), (21, 22)) matrix, (11, 12) and (21, 22) are matrix rows.

Special Types

Special data types can be used to represent:

  • A mixed data collection (List).
  • A set of named values of different types (Dict).
  • An existing file, or a file that has to be created in a specified place, or a temporary file (File).
  • Arbitrary byte sequence, binary data (Blob).

List is an heterogeneous ordered sequence: elements may have different types and are addressed by index. An element may have any base or special type (only Undefined is not allowed). Indices are zero-based.

Dict is an heterogeneous associative (unordered) array containing key-value pairs. Values may have different types and are addressed by keys. A value may have any base or special type (only Undefined is not allowed). Keys may be strings only.

File is an advanced type for file handling (a file-like object). It is used to pass control over files between blocks. For details on File properties, see the File block page.

Blob is a binary array. This type is introduced to support Python bytearray type.

Conversion Rules

Two cases have to be considered to understand data type conversion in pSeven. The first is when data is converted between pSeven types — for example, it travels through a link connecting two block ports with different types. The second is conversion between a pSeven type and a native Python type. It occurs, for example, in blocks that support Python scripting. When a value received to port is assigned to a script variable, it is converted from a pSeven type to a Python type. When the block reads a value of a script variable in order to output it to a port, this value is converted from a Python type to a pSeven type.

pSeven Types Conversion

pSeven allows conversion of base data types to each other, if it does not lead to data loss:

  • Scalars can be converted to vectors and matrices (naturally, the result contains a single element).
  • Vectors can be converted to single-row matrices. Also, a vector can be converted to a scalar, but only if the vector contains a single element.
  • Matrices can be converted to vectors if they contain a single row or a single column, and to scalars if they contain a single element.
  • “Inner” type conversion is possible according to the following order: Boolean, integer, real, string.

For example, a single-element IntMatrix can be converted to a RealScalar (((42)) to 42.0), but not to a BoolMatrix because the value of 42 will be lost. A single-column RealMatrix can be converted to a StringVector (((1.0), (2.0), (3.0)) to ("1.0", "2.0", "3.0")) or RealVector, but not to a StringScalar because it cannot describe the structure.

Warning

Converting floating point numbers to strings reduces precision.

In some situations there are two or more possible conversions. For example, a link may connect an output port of IntVector type with an input port that accepts both IntMatrix and RealVector. In such cases the preferred conversion is selected in the following order: BoolScalar, BoolVector, BoolMatrix, IntScalar, IntVector, IntMatrix, RealScalar, RealVector, RealMatrix, StringScalar, StringVector, StringMatrix. So the integer vector in the example above is converted to an integer matrix with a single row.

All base data types can also be converted to List. A scalar becomes a List with a single element, a vector becomes a List of scalars, and a matrix is split by rows and becomes a list of vectors.

A homogeneous flat List (a list containing only scalar values of the same type) can be converted to a vector — for example, [1.0, 2.0, 3.0] to RealVector (1.0, 2.0, 3.0). Such lists can also be directly converted to single-row matrices.

Similarly, a List that contains only vectors of the same dimension (or List values that can be converted to such vectors) can be converted to a matrix. That is, if the List can be interpreted as a rectangular matrix, it can be converted to a matrix. For example, a List of integer vectors [(1, 2), (3, 4)] can be converted to IntMatrix ((1, 2), (3, 4)), as can be a List of equally sized lists of integers [[1, 2], [3, 4]].

Dict, File, and Blob can not be converted to or from any other type except their trivial conversion to List, which results in a List containing a single element.

pSeven type descriptions in the pSeven Data Types reference include allowed conversions between pSeven data types.

Python to pSeven Types Conversion

pSeven supports bool, int and float Python types, None values, str, unicode, list, tuple and bytearray sequences, dict and numpy.ndarray.

When converting from Python to pSeven types, Boolean, numeric, and string values by default are converted to appropriate scalars: bool to BoolScalar, int to IntScalar (long is not supported), float to RealScalar, str and unicode to StringScalar. It is also possible to force converting them to a vector or matrix type.

Flat list or tuple is by default converted to an appropriate vector type, provided that:

  • all elements are of the same type, and
  • this type is bool, int, float, str, or unicode.

A list or tuple like this can also be forcibly converted to a single-row matrix.

Nested list or tuple is by default converted to a nested List, and can be forcibly converted to a matrix, provided that:

  • the top level contains only lists and/or tuples,
  • nesting level is 1,
  • all nested lists (tuples) are of the same length,
  • all elements of the nested lists (tuples) are of the same type, and
  • this type is bool, int, float, str, or unicode.

Other tuples and lists (containing different types, or with various nesting levels) can be converted to List only. Contained types must be supported by pSeven (for example, a List of complex numbers is not possible).

When there are two or more possible conversions from a Python to a pSeven type, the rule is the same as for pSeven types (see section pSeven Types Conversion). For example, if an int value has to be output to a port that allows IntVector and RealScalar, it is converted to IntVector with a single element because or the preferred conversion order (from BoolScalar to StringMatrix, see above). Note that the result of conversion is not RealScalar despite logically the original value is scalar. In case a list or tuple is output to a port with multiple types, pSeven again tries preferred conversions or, if they all fail, tries converting to List. A few examples of list conversion are:

Python bytearray is converted to Blob by default. The only other possible conversion is bytearray to List (resulting List will contain a single Blob element).

Dictionaries (dict) are converted to Dict if all keys are strings, otherwise it produces a type error.

None is always converted to Undefined.

An N-dimensional array (numpy.ndarray) of values of the same type is by default converted to vector if N = 1 and to matrix if N = 2 (1-dimensional array can be forcibly converted to a single-row matrix). Any other numpy.ndarray is converted to List (note that array elements must be of a supported type).

pSeven type descriptions in the pSeven Data Types reference include Python equivalents to pSeven types. For a Python type, being equivalent to a pSeven type means that this Python type is converted to this pSeven type by default (that is, if conversion is not restricted by a port type or other means).

pSeven to Python Types Conversion

Rules of conversion from pSeven to Python types are the most strict compared to conversion between pSeven types or from Python to pSeven types. This conversion is controlled by pSeven, and there is no way to force convert some pSeven type to a non-default Python type.

  • BoolScalar, IntScalar, RealScalar, and StringScalar are converted to bool, int, float and unicode respectively.
  • A vector type is converted to 1-dimensional numpy.ndarray. This array contains values converted from pSeven scalar type to Python type (see above).
  • A matrix type is converted to 2-dimensional numpy.ndarray. This array contains values converted from pSeven scalar type to Python type (see above).
  • List is converted to list. List elements are converted following the rules for their types.
  • Dict is converted to dict. Dict keys are converted to unicode (as they are originally StringScalar). Dict values are converted following the rules for their types.
  • Blob is converted to bytearray.
  • File cannot be converted to a Python file-like object or another type directly.
  • Undefined is converted to None.

pSeven type descriptions in the pSeven Data Types reference include Python equivalents to pSeven types. Considering pSeven to Python types conversion, the Python equivalent is the resulting type.