Representations of Values

Python's strings, byte strings, integers, booleans, and double-precision floats stand directly for their Preserves counterparts. Wrapper objects for Float and Symbol complete the suite of atomic types.

Python's lists and tuples correspond to Preserves Sequences, and dicts and sets to Dictionary and Set values, respectively. Preserves Records are represented by Record objects. Finally, embedded values are represented by Embedded objects.

The preserves.values module implements the core representations of Preserves Values as Python values.

Annotated(item)

Bases: object

A Preserves Value along with a sequence of Values annotating it. Compares equal to the underlying Value, ignoring the annotations. See the specification document for more about annotations.

>>> import preserves
>>> a = preserves.parse('''
... # A comment
... [1 2 3]
... ''', include_annotations=True)
>>> a
@'A comment' (1, 2, 3)
>>> a.item
(1, 2, 3)
>>> a.annotations
['A comment']
>>> a == (1, 2, 3)
True
>>> a == preserves.parse('@xyz [1 2 3]', include_annotations=True)
True
>>> a[0]
Traceback (most recent call last):
  ...
TypeError: 'Annotated' object is not subscriptable
>>> a.item[0]
1
>>> type(a.item[0])
<class 'preserves.values.Annotated'>
>>> a.item[0].annotations
[]
>>> print(preserves.stringify(a))
@"A comment" [1 2 3]
>>> print(preserves.stringify(a, include_annotations=False))
[1 2 3]

Attributes:

Name	Type	Description
item	Value	the underlying annotated Value
annotations	list[Value]	the annotations attached to self.item

Source code in preserves/values.py

def __init__(self, item):
    self.annotations = []
    self.item = item

peel()

Calls strip_annotations on self with depth=1.

Source code in preserves/values.py

def peel(self):
    """Calls [strip_annotations][preserves.values.strip_annotations] on `self` with `depth=1`."""
    return strip_annotations(self, 1)

strip(depth=inf)

Calls strip_annotations on self and depth.

Source code in preserves/values.py

def strip(self, depth=inf):
    """Calls [strip_annotations][preserves.values.strip_annotations] on `self` and `depth`."""
    return strip_annotations(self, depth)

Embedded(embeddedValue)

Representation of a Preserves Embedded value. For more on the meaning and use of embedded values, see the specification.

>>> import io
>>> e = Embedded(io.StringIO('some text'))
>>> e                                        # doctest: +ELLIPSIS
#!<_io.StringIO object at ...>
>>> e.embeddedValue                          # doctest: +ELLIPSIS
<_io.StringIO object at ...>

>>> import preserves
>>> print(preserves.stringify(Embedded(None)))
Traceback (most recent call last):
  ...
TypeError: Cannot preserves-format: None
>>> print(preserves.stringify(Embedded(None), format_embedded=lambda x: 'abcdef'))
#!"abcdef"

Attributes:

Name	Type	Description
embeddedValue		any Python value; could be a platform object, could be a representation of a Preserves Value, could be None, could be anything!

Source code in preserves/values.py

def __init__(self, embeddedValue):
    self.embeddedValue = embeddedValue

Float(value)

Bases: object

Wrapper for treating a Python double-precision floating-point value as a single-precision (32-bit) float, from Preserves' perspective. (Python lacks native single-precision floating point support.)

>>> Float(3.45)
Float(3.45)
>>> import preserves
>>> preserves.stringify(Float(3.45))
'3.45f'
>>> preserves.stringify(3.45)
'3.45'
>>> preserves.parse('3.45f')
Float(3.45)
>>> preserves.parse('3.45')
3.45
>>> preserves.encode(Float(3.45))
b'\x87\x04@\\\xcc\xcd'
>>> preserves.encode(3.45)
b'\x87\x08@\x0b\x99\x99\x99\x99\x99\x9a'

Attributes:

Name	Type	Description
value	float	the double-precision representation of intended single-precision value

Source code in preserves/values.py

def __init__(self, value):
    self.value = value

from_bytes(bs) staticmethod

Converts a 4-byte-long byte string to a 32-bit single-precision floating point value wrapped in a Float instance. Takes care to preserve the quiet/signalling bit-pattern of NaN values, unlike its struct.unpack('>f', ...) equivalent.

>>> Float.from_bytes(b'\x7f\x80\x00{')
Float(nan)
>>> Float.from_bytes(b'\x7f\x80\x00{').to_bytes()
b'\x7f\x80\x00{'

>>> struct.unpack('>f', b'\x7f\x80\x00{')[0]
nan
>>> Float(struct.unpack('>f', b'\x7f\x80\x00{')[0]).to_bytes()
b'\x7f\xc0\x00{'
>>> struct.pack('>f', struct.unpack('>f', b'\x7f\x80\x00{')[0])
b'\x7f\xc0\x00{'

(Note well the difference between 7f80007b and 7fc0007b!)

Source code in preserves/values.py

@staticmethod
def from_bytes(bs):
    """Converts a 4-byte-long byte string to a 32-bit single-precision floating point value
    wrapped in a [Float][preserves.values.Float] instance. Takes care to preserve the
    quiet/signalling bit-pattern of NaN values, unlike its `struct.unpack('>f', ...)`
    equivalent.

    ```python
    >>> Float.from_bytes(b'\\x7f\\x80\\x00{')
    Float(nan)
    >>> Float.from_bytes(b'\\x7f\\x80\\x00{').to_bytes()
    b'\\x7f\\x80\\x00{'

    >>> struct.unpack('>f', b'\\x7f\\x80\\x00{')[0]
    nan
    >>> Float(struct.unpack('>f', b'\\x7f\\x80\\x00{')[0]).to_bytes()
    b'\\x7f\\xc0\\x00{'
    >>> struct.pack('>f', struct.unpack('>f', b'\\x7f\\x80\\x00{')[0])
    b'\\x7f\\xc0\\x00{'

    ```

    (Note well the difference between `7f80007b` and `7fc0007b`!)

    """
    vf = struct.unpack('>I', bs)[0]
    if (vf & 0x7f800000) == 0x7f800000:
        # NaN or inf. Preserve quiet/signalling bit by manually expanding to double-precision.
        sign = vf >> 31
        payload = vf & 0x007fffff
        dbs = struct.pack('>Q', (sign << 63) | 0x7ff0000000000000 | (payload << 29))
        return Float(struct.unpack('>d', dbs)[0])
    else:
        return Float(struct.unpack('>f', bs)[0])

to_bytes()

Converts this 32-bit single-precision floating point value to its binary32 format, taking care to preserve the quiet/signalling bit-pattern of NaN values, unlike its struct.pack('>f', ...) equivalent.

>>> Float.from_bytes(b'\x7f\x80\x00{')
Float(nan)
>>> Float.from_bytes(b'\x7f\x80\x00{').to_bytes()
b'\x7f\x80\x00{'

>>> struct.unpack('>f', b'\x7f\x80\x00{')[0]
nan
>>> Float(struct.unpack('>f', b'\x7f\x80\x00{')[0]).to_bytes()
b'\x7f\xc0\x00{'
>>> struct.pack('>f', struct.unpack('>f', b'\x7f\x80\x00{')[0])
b'\x7f\xc0\x00{'

(Note well the difference between 7f80007b and 7fc0007b!)

Source code in preserves/values.py

def to_bytes(self):
    """Converts this 32-bit single-precision floating point value to its binary32 format,
    taking care to preserve the quiet/signalling bit-pattern of NaN values, unlike its
    `struct.pack('>f', ...)` equivalent.

    ```python
    >>> Float.from_bytes(b'\\x7f\\x80\\x00{')
    Float(nan)
    >>> Float.from_bytes(b'\\x7f\\x80\\x00{').to_bytes()
    b'\\x7f\\x80\\x00{'

    >>> struct.unpack('>f', b'\\x7f\\x80\\x00{')[0]
    nan
    >>> Float(struct.unpack('>f', b'\\x7f\\x80\\x00{')[0]).to_bytes()
    b'\\x7f\\xc0\\x00{'
    >>> struct.pack('>f', struct.unpack('>f', b'\\x7f\\x80\\x00{')[0])
    b'\\x7f\\xc0\\x00{'

    ```

    (Note well the difference between `7f80007b` and `7fc0007b`!)

    """

    if math.isnan(self.value) or math.isinf(self.value):
        dbs = struct.pack('>d', self.value)
        vd = struct.unpack('>Q', dbs)[0]
        sign = vd >> 63
        payload = (vd >> 29) & 0x007fffff
        vf = (sign << 31) | 0x7f800000 | payload
        return struct.pack('>I', vf)
    else:
        return struct.pack('>f', self.value)

ImmutableDict(*args, **kwargs)

Bases: dict

A subclass of Python's built-in dict that overrides methods that could mutate the dictionary, causing them to raise TypeError('Immutable') if called.

Implements the __hash__ method, allowing ImmutableDict instances to be used whereever immutable data are permitted; in particular, as keys in other dictionaries.

>>> d = ImmutableDict([('a', 1), ('b', 2)])
>>> d
{'a': 1, 'b': 2}
>>> d['c'] = 3
Traceback (most recent call last):
  ...
TypeError: Immutable
>>> del d['b']
Traceback (most recent call last):
  ...
TypeError: Immutable

Source code in preserves/values.py

def __init__(self, *args, **kwargs):
    if hasattr(self, '__hash'): raise TypeError('Immutable')
    super(ImmutableDict, self).__init__(*args, **kwargs)
    self.__hash = None

from_kvs(kvs) staticmethod

Constructs an ImmutableDict from a sequence of alternating keys and values; compare to the ImmutableDict constructor, which takes a sequence of key-value pairs.

>>> ImmutableDict.from_kvs(['a', 1, 'b', 2])
{'a': 1, 'b': 2}
>>> ImmutableDict.from_kvs(['a', 1, 'b', 2])['c'] = 3
Traceback (most recent call last):
  ...
TypeError: Immutable

Source code in preserves/values.py

@staticmethod
def from_kvs(kvs):
    """Constructs an [ImmutableDict][preserves.values.ImmutableDict] from a sequence of
    alternating keys and values; compare to the
    [ImmutableDict][preserves.values.ImmutableDict] constructor, which takes a sequence of
    key-value pairs.

    ```python
    >>> ImmutableDict.from_kvs(['a', 1, 'b', 2])
    {'a': 1, 'b': 2}
    >>> ImmutableDict.from_kvs(['a', 1, 'b', 2])['c'] = 3
    Traceback (most recent call last):
      ...
    TypeError: Immutable

    ```

    """

    i = iter(kvs)
    result = ImmutableDict()
    result_proxy = super(ImmutableDict, result)
    try:
        while True:
            k = next(i)
            try:
                v = next(i)
            except StopIteration:
                raise DecodeError("Missing dictionary value")
            if k in result:
                raise DecodeError("Duplicate key: " + repr(k))
            result_proxy.__setitem__(k, v)
    except StopIteration:
        pass
    return result

Record(key, fields)

Bases: object

Representation of Preserves Records, which are a pair of a label Value and a sequence of field Values.

>>> r = Record(Symbol('label'), ['field1', ['field2item1', 'field2item2']])
>>> r
#label('field1', ['field2item1', 'field2item2'])
>>> r.key
#label
>>> r.fields
('field1', ['field2item1', 'field2item2'])
>>> import preserves
>>> preserves.stringify(r)
'<label "field1" ["field2item1" "field2item2"]>'
>>> r == preserves.parse('<label "field1" ["field2item1" "field2item2"]>')
True

Parameters:

Name	Type	Description	Default
key	Value	the Record's label	required
fields	iterable[Value]	the fields of the Record	required

Attributes:

Name	Type	Description
key	Value	the Record's label
fields	tuple[Value]	the fields of the Record

Source code in preserves/values.py

def __init__(self, key, fields):
    self.key = key
    self.fields = tuple(fields)
    self.__hash = None

makeBasicConstructor(label, fieldNames) staticmethod

Constructs and returns a "constructor" for Records having a certain label and number of fields.

Deprecated

Use preserves.schema definitions instead.

The "constructor" is a callable function that accepts len(fields) arguments and returns a Record with label as its label and the arguments to the constructor as field values.

In addition, the "constructor" has a constructorInfo attribute holding a RecordConstructorInfo object, an isClassOf attribute holding a unary function that returns True iff its argument is a Record with label label and arity len(fieldNames), and an ensureClassOf attribute that raises an Exception if isClassOf returns false on its argument and returns the argument otherwise.

Finally, for each field name f in fieldNames, the "constructor" object has an attribute _f that is a unary function that retrieves the f field from the passed in argument.

>>> c = Record.makeBasicConstructor(Symbol('date'), 'year month day')
>>> c(1969, 7, 16)
#date(1969, 7, 16)
>>> c.constructorInfo
#date/3
>>> c.isClassOf(c(1969, 7, 16))
True
>>> c.isClassOf(Record(Symbol('date'), [1969, 7, 16]))
True
>>> c.isClassOf(Record(Symbol('date'), [1969]))
False
>>> c.ensureClassOf(c(1969, 7, 16))
#date(1969, 7, 16)
>>> c.ensureClassOf(Record(Symbol('date'), [1969]))
Traceback (most recent call last):
  ...
TypeError: Record: expected #date/3, got #date(1969)
>>> c._year(c(1969, 7, 16))
1969
>>> c._month(c(1969, 7, 16))
7
>>> c._day(c(1969, 7, 16))
16

Parameters:

Name	Type	Description	Default
label	Value	Label to use for constructed/matched Records	required
fieldNames	tuple[str] | list[str] | str	Names of the Record's fields	required

Source code in preserves/values.py

@staticmethod
def makeBasicConstructor(label, fieldNames):
    """Constructs and returns a "constructor" for `Record`s having a certain `label` and
    number of fields.

    Deprecated:
       Use [preserves.schema][] definitions instead.

    The "constructor" is a callable function that accepts `len(fields)` arguments and
    returns a [Record][preserves.values.Record] with `label` as its label and the arguments
    to the constructor as field values.

    In addition, the "constructor" has a `constructorInfo` attribute holding a
    [RecordConstructorInfo][preserves.values.RecordConstructorInfo] object, an `isClassOf`
    attribute holding a unary function that returns `True` iff its argument is a
    [Record][preserves.values.Record] with label `label` and arity `len(fieldNames)`, and
    an `ensureClassOf` attribute that raises an `Exception` if `isClassOf` returns false on
    its argument and returns the argument otherwise.

    Finally, for each field name `f` in `fieldNames`, the "constructor" object has an
    attribute `_f` that is a unary function that retrieves the `f` field from the passed in
    argument.

    ```python
    >>> c = Record.makeBasicConstructor(Symbol('date'), 'year month day')
    >>> c(1969, 7, 16)
    #date(1969, 7, 16)
    >>> c.constructorInfo
    #date/3
    >>> c.isClassOf(c(1969, 7, 16))
    True
    >>> c.isClassOf(Record(Symbol('date'), [1969, 7, 16]))
    True
    >>> c.isClassOf(Record(Symbol('date'), [1969]))
    False
    >>> c.ensureClassOf(c(1969, 7, 16))
    #date(1969, 7, 16)
    >>> c.ensureClassOf(Record(Symbol('date'), [1969]))
    Traceback (most recent call last):
      ...
    TypeError: Record: expected #date/3, got #date(1969)
    >>> c._year(c(1969, 7, 16))
    1969
    >>> c._month(c(1969, 7, 16))
    7
    >>> c._day(c(1969, 7, 16))
    16

    ```

    Args:
        label (Value): Label to use for constructed/matched `Record`s
        fieldNames (tuple[str] | list[str] | str): Names of the `Record`'s fields

    """
    if type(fieldNames) == str:
        fieldNames = fieldNames.split()
    arity = len(fieldNames)
    def ctor(*fields):
        if len(fields) != arity:
            raise Exception("Record: cannot instantiate %r expecting %d fields with %d fields"%(
                label,
                arity,
                len(fields)))
        return Record(label, fields)
    ctor.constructorInfo = RecordConstructorInfo(label, arity)
    ctor.isClassOf = lambda v: \
                     isinstance(v, Record) and v.key == label and len(v.fields) == arity
    def ensureClassOf(v):
        if not ctor.isClassOf(v):
            raise TypeError("Record: expected %r/%d, got %r" % (label, arity, v))
        return v
    ctor.ensureClassOf = ensureClassOf
    for fieldIndex in range(len(fieldNames)):
        fieldName = fieldNames[fieldIndex]
        # Stupid python scoping bites again
        def getter(fieldIndex):
            return lambda v: ensureClassOf(v)[fieldIndex]
        setattr(ctor, '_' + fieldName, getter(fieldIndex))
    return ctor

makeConstructor(labelSymbolText, fieldNames) staticmethod

Equivalent to Record.makeBasicConstructor(Symbol(labelSymbolText), fieldNames).

Deprecated

Use preserves.schema definitions instead.

Source code in preserves/values.py

@staticmethod
def makeConstructor(labelSymbolText, fieldNames):
    """
    Equivalent to `Record.makeBasicConstructor(Symbol(labelSymbolText), fieldNames)`.

    Deprecated:
       Use [preserves.schema][] definitions instead.
    """
    return Record.makeBasicConstructor(Symbol(labelSymbolText), fieldNames)

RecordConstructorInfo(key, arity)

Bases: object

Describes the shape of a Record constructor, namely its label and its arity (field count).

>>> RecordConstructorInfo(Symbol('label'), 3)
#label/3

Attributes:

Name	Type	Description
key	Value	the label of matching Records
arity	int	the number of fields in matching Records

Source code in preserves/values.py

def __init__(self, key, arity):
    self.key = key
    self.arity = arity

Symbol(name)

Bases: object

Representation of Preserves Symbols.

>>> Symbol('xyz')
#xyz
>>> Symbol('xyz').name
'xyz'
>>> repr(Symbol('xyz'))
'#xyz'
>>> str(Symbol('xyz'))
'xyz'
>>> import preserves
>>> preserves.stringify(Symbol('xyz'))
'xyz'
>>> preserves.stringify(Symbol('hello world'))
'|hello world|'
>>> preserves.parse('xyz')
#xyz
>>> preserves.parse('|hello world|')
#hello world

Attributes:

Name	Type	Description
name	str | Symbol	The symbol's text label. If an existing Symbol is passed in, the existing Symbol's name is used as the name for the new Symbol.

Source code in preserves/values.py

def __init__(self, name):
    self.name = name.name if isinstance(name, Symbol) else name

annotate(v, *anns)

Wraps v in an Annotated object, if it isn't already wrapped, and appends each of the anns to the Annotated's annotations sequence. NOTE: Does not recursively ensure that any parts of the argument v are themselves wrapped in Annotated objects!

>>> import preserves
>>> print(preserves.stringify(annotate(123, "A comment", "Another comment")))
@"A comment" @"Another comment" 123

Source code in preserves/values.py

def annotate(v, *anns):
    """Wraps `v` in an [Annotated][preserves.values.Annotated] object, if it isn't already
    wrapped, and appends each of the `anns` to the [Annotated][preserves.values.Annotated]'s
    `annotations` sequence. NOTE: Does not recursively ensure that any parts of the argument
    `v` are themselves wrapped in [Annotated][preserves.values.Annotated] objects!

    ```python
    >>> import preserves
    >>> print(preserves.stringify(annotate(123, "A comment", "Another comment")))
    @"A comment" @"Another comment" 123

    ```
    """
    if not is_annotated(v):
        v = Annotated(v)
    for a in anns:
        v.annotations.append(a)
    return v

cmp_floats(a, b)

Implements the totalOrder predicate defined in section 5.10 of IEEE Std 754-2008.

Source code in preserves/values.py

def cmp_floats(a, b):
    """Implements the `totalOrder` predicate defined in section 5.10 of [IEEE Std
    754-2008](https://dx.doi.org/10.1109/IEEESTD.2008.4610935).

    """
    a = float_to_int(a)
    b = float_to_int(b)
    if a & 0x8000000000000000: a = a ^ 0x7fffffffffffffff
    if b & 0x8000000000000000: b = b ^ 0x7fffffffffffffff
    return a - b

dict_kvs(d)

Generator function yielding a sequence of alternating keys and values from d. In some sense the inverse of ImmutableDict.from_kvs.

>>> list(dict_kvs({'a': 1, 'b': 2}))
['a', 1, 'b', 2]

Source code in preserves/values.py

def dict_kvs(d):
    """Generator function yielding a sequence of alternating keys and values from `d`. In some
    sense the inverse of [ImmutableDict.from_kvs][preserves.values.ImmutableDict.from_kvs].

    ```python
    >>> list(dict_kvs({'a': 1, 'b': 2}))
    ['a', 1, 'b', 2]

    ```
    """
    for k in d:
        yield k
        yield d[k]

is_annotated(v)

True iff v is an instance of Annotated.

Source code in preserves/values.py

def is_annotated(v):
    """`True` iff `v` is an instance of [Annotated][preserves.values.Annotated]."""
    return isinstance(v, Annotated)

preserve(v)

Converts v to a representation of a Preserves Value by (repeatedly) setting

v = v.__preserve__()

while v has a __preserve__ method. Parsed Schema values are able to render themselves to their serialized representations this way.

Source code in preserves/values.py

def preserve(v):
    """Converts `v` to a representation of a Preserves `Value` by (repeatedly) setting

    ```python
    v = v.__preserve__()
    ```

    while `v` has a `__preserve__` method. Parsed [Schema][preserves.schema]
    values are able to render themselves to their serialized representations this way.

    """
    while hasattr(v, '__preserve__'):
        v = v.__preserve__()
    return v

strip_annotations(v, depth=inf)

Exposes depth layers of raw structure of potentially-Annotated Values. If depth==0 or v is not Annotated, just returns v. Otherwise, descends recursively into the structure of v.item.

>>> import preserves
>>> a = preserves.parse('@"A comment" [@a 1 @b 2 @c 3]', include_annotations=True)
>>> is_annotated(a)
True
>>> print(preserves.stringify(a))
@"A comment" [@a 1 @b 2 @c 3]
>>> print(preserves.stringify(strip_annotations(a)))
[1 2 3]
>>> print(preserves.stringify(strip_annotations(a, depth=1)))
[@a 1 @b 2 @c 3]

Source code in preserves/values.py

def strip_annotations(v, depth=inf):
    """Exposes `depth` layers of raw structure of
    potentially-[Annotated][preserves.values.Annotated] `Value`s. If `depth==0` or `v` is not
    [Annotated][preserves.values.Annotated], just returns `v`. Otherwise, descends recursively
    into the structure of `v.item`.

    ```python
    >>> import preserves
    >>> a = preserves.parse('@"A comment" [@a 1 @b 2 @c 3]', include_annotations=True)
    >>> is_annotated(a)
    True
    >>> print(preserves.stringify(a))
    @"A comment" [@a 1 @b 2 @c 3]
    >>> print(preserves.stringify(strip_annotations(a)))
    [1 2 3]
    >>> print(preserves.stringify(strip_annotations(a, depth=1)))
    [@a 1 @b 2 @c 3]

    ```
    """

    if depth == 0: return v
    if not is_annotated(v): return v

    next_depth = depth - 1
    def walk(v):
        return strip_annotations(v, next_depth)

    v = v.item
    if isinstance(v, Record):
        return Record(strip_annotations(v.key, depth), tuple(walk(f) for f in v.fields))
    elif isinstance(v, list):
        return tuple(walk(f) for f in v)
    elif isinstance(v, tuple):
        return tuple(walk(f) for f in v)
    elif isinstance(v, set):
        return frozenset(walk(f) for f in v)
    elif isinstance(v, frozenset):
        return frozenset(walk(f) for f in v)
    elif isinstance(v, dict):
        return ImmutableDict.from_kvs(walk(f) for f in dict_kvs(v))
    elif is_annotated(v):
        raise ValueError('Improper annotation structure')
    else:
        return v

---

Last update: March 16, 2023
Created: March 16, 2023