This section introduces the glib.variant.VariantG type system. It is based, in
large part, on the D-Bus type system, with two major changes and
some minor lifting of restrictions. The
D-Bus specification,
therefore, provides a significant amount of
information that is useful when working with glib.variant.VariantG.
The first major change with respect to the D-Bus type system is the
introduction of maybe (or ‘nullable’) types. Any type in glib.variant.VariantG
can be converted to a maybe type, in which case, nothing (or null)
becomes a valid value. Maybe types have been added by introducing the
character m to type strings.
The second major change is that the glib.variant.VariantG type system supports
the concept of ‘indefinite types’ — types that are less specific than
the normal types found in D-Bus. For example, it is possible to speak
of ‘an array of any type’ in glib.variant.VariantG, where the D-Bus type system
would require you to speak of ‘an array of integers’ or ‘an array of
strings’. Indefinite types have been added by introducing the
characters *, ? and r to type strings.
Finally, all arbitrary restrictions relating to the complexity of
types are lifted along with the restriction that dictionary entries
may only appear nested inside of arrays.
Just as in D-Bus, glib.variant.VariantG types are described with strings (‘type
strings’). Subject to the differences mentioned above, these strings
are of the same form as those found in D-Bus. Note, however: D-Bus
always works in terms of messages and therefore individual type
strings appear nowhere in its interface. Instead, ‘signatures’
are a concatenation of the strings of the type of each argument in a
message. glib.variant.VariantG deals with single values directly so
glib.variant.VariantG type strings always describe the type of exactly one
value. This means that a D-Bus signature string is generally not a valid
glib.variant.VariantG type string — except in the case that it is the signature
of a message containing exactly one argument.
An indefinite type is similar in spirit to what may be called an
abstract type in other type systems. No value can exist that has an
indefinite type as its type, but values can exist that have types
that are subtypes of indefinite types. That is to say,
glib.variant.VariantG.getType will never return an indefinite type, but
calling glib.variant.VariantG.isOfType with an indefinite type may return
true. For example, you cannot have a value that represents ‘an
array of no particular type’, but you can have an ‘array of integers’
which certainly matches the type of ‘an array of no particular type’,
since ‘array of integers’ is a subtype of ‘array of no particular
type’.
one of the characters a or m, followed by another type string
the character (, followed by a concatenation of zero or more other
type strings, followed by the character )
the character {, followed by a basic type string (see below),
followed by another type string, followed by the character }
A basic type string describes a basic type (as per
glib.variant_type.VariantType.isBasic) and is always a single character in
length. The valid basic type strings are b, y, n, q, i, u, x,
t, h, d, s, o, g and ?.
The above definition is recursive to arbitrary depth. aaaaai and
(ui(nq((y)))s) are both valid type strings, as is
a(aa(ui)(qna{ya(yd)})). In order to not hit memory limits,
glib.variant.VariantG imposes a limit on recursion depth of 65 nested
containers. This is the limit in the D-Bus specification (64) plus one to
allow a [gio.dbus_message.DBusMessage] to be nested in
a top-level tuple.
The meaning of each of the characters is as follows:
b: the type string of G_VARIANT_TYPE_BOOLEAN; a boolean value.
y: the type string of G_VARIANT_TYPE_BYTE; a byte.
n: the type string of G_VARIANT_TYPE_INT16; a signed 16 bit integer.
q: the type string of G_VARIANT_TYPE_UINT16; an unsigned 16 bit integer.
i: the type string of G_VARIANT_TYPE_INT32; a signed 32 bit integer.
u: the type string of G_VARIANT_TYPE_UINT32; an unsigned 32 bit integer.
x: the type string of G_VARIANT_TYPE_INT64; a signed 64 bit integer.
t: the type string of G_VARIANT_TYPE_UINT64; an unsigned 64 bit integer.
h: the type string of G_VARIANT_TYPE_HANDLE; a signed 32 bit value
that, by convention, is used as an index into an array of file
descriptors that are sent alongside a D-Bus message.
d: the type string of G_VARIANT_TYPE_DOUBLE; a double precision
floating point value.
s: the type string of G_VARIANT_TYPE_STRING; a string.
o: the type string of G_VARIANT_TYPE_OBJECT_PATH; a string in the form
of a D-Bus object path.
g: the type string of G_VARIANT_TYPE_SIGNATURE; a string in the form of
a D-Bus type signature.
?: the type string of G_VARIANT_TYPE_BASIC; an indefinite type that
is a supertype of any of the basic types.
v: the type string of G_VARIANT_TYPE_VARIANT; a container type that
contain any other type of value.
a: used as a prefix on another type string to mean an array of that
type; the type string ai, for example, is the type of an array of
signed 32-bit integers.
m: used as a prefix on another type string to mean a ‘maybe’, or
‘nullable’, version of that type; the type string ms, for example,
is the type of a value that maybe contains a string, or maybe contains
nothing.
(): used to enclose zero or more other concatenated type strings to
create a tuple type; the type string (is), for example, is the type of
a pair of an integer and a string.
r: the type string of G_VARIANT_TYPE_TUPLE; an indefinite type that is
a supertype of any tuple type, regardless of the number of items.
{}: used to enclose a basic type string concatenated with another type
string to create a dictionary entry type, which usually appears inside of
an array to form a dictionary; the type string a{sd}, for example, is
the type of a dictionary that maps strings to double precision floating
point values.
The first type (the basic type) is the key type and the second type is
the value type. The reason that the first type is restricted to being a
basic type is so that it can easily be hashed.
*: the type string of G_VARIANT_TYPE_ANY; the indefinite type that is
a supertype of all types. Note that, as with all type strings, this
character represents exactly one type. It cannot be used inside of tuples
to mean ‘any number of items’.
Any type string of a container that contains an indefinite type is,
itself, an indefinite type. For example, the type string a*
(corresponding to G_VARIANT_TYPE_ARRAY) is an indefinite type
that is a supertype of every array type. (*s) is a supertype
of all tuples that contain exactly two items where the second
item is a string.
a{?*} is an indefinite type that is a supertype of all arrays
containing dictionary entries where the key is any basic type and
the value is any type at all. This is, by definition, a dictionary,
so this type string corresponds to G_VARIANT_TYPE_DICTIONARY. Note
that, due to the restriction that the key of a dictionary entry must
be a basic type, {**} is not a valid type string.
A type in the glib.variant.VariantG type system.
This section introduces the glib.variant.VariantG type system. It is based, in large part, on the D-Bus type system, with two major changes and some minor lifting of restrictions. The D-Bus specification, therefore, provides a significant amount of information that is useful when working with glib.variant.VariantG.
The first major change with respect to the D-Bus type system is the introduction of maybe (or ‘nullable’) types. Any type in glib.variant.VariantG can be converted to a maybe type, in which case, nothing (or null) becomes a valid value. Maybe types have been added by introducing the character m to type strings.
The second major change is that the glib.variant.VariantG type system supports the concept of ‘indefinite types’ — types that are less specific than the normal types found in D-Bus. For example, it is possible to speak of ‘an array of any type’ in glib.variant.VariantG, where the D-Bus type system would require you to speak of ‘an array of integers’ or ‘an array of strings’. Indefinite types have been added by introducing the characters *, ? and r to type strings.
Finally, all arbitrary restrictions relating to the complexity of types are lifted along with the restriction that dictionary entries may only appear nested inside of arrays.
Just as in D-Bus, glib.variant.VariantG types are described with strings (‘type strings’). Subject to the differences mentioned above, these strings are of the same form as those found in D-Bus. Note, however: D-Bus always works in terms of messages and therefore individual type strings appear nowhere in its interface. Instead, ‘signatures’ are a concatenation of the strings of the type of each argument in a message. glib.variant.VariantG deals with single values directly so glib.variant.VariantG type strings always describe the type of exactly one value. This means that a D-Bus signature string is generally not a valid glib.variant.VariantG type string — except in the case that it is the signature of a message containing exactly one argument.
An indefinite type is similar in spirit to what may be called an abstract type in other type systems. No value can exist that has an indefinite type as its type, but values can exist that have types that are subtypes of indefinite types. That is to say, glib.variant.VariantG.getType will never return an indefinite type, but calling glib.variant.VariantG.isOfType with an indefinite type may return true. For example, you cannot have a value that represents ‘an array of no particular type’, but you can have an ‘array of integers’ which certainly matches the type of ‘an array of no particular type’, since ‘array of integers’ is a subtype of ‘array of no particular type’.
This is similar to how instances of abstract classes may not directly exist in other type systems, but instances of their non-abstract subtypes may. For example, in GTK, no object that has the type of [gtk.widget.Widget] can exist (since gtk.widget.Widget is an abstract class), but a [gtk.window.Window] can certainly be instantiated, and you would say that a gtk.window.Window is a gtk.widget.Widget (since gtk.window.Window is a subclass of gtk.widget.Widget).
Two types may not be compared by value; use glib.variant_type.VariantType.equal or glib.variant_type.VariantType.isSubtypeOf May be copied using glib.variant_type.VariantType.copy and freed using glib.variant_type.VariantType.free.
GVariant Type Strings
A glib.variant.VariantG type string can be any of the following:
A basic type string describes a basic type (as per glib.variant_type.VariantType.isBasic) and is always a single character in length. The valid basic type strings are b, y, n, q, i, u, x, t, h, d, s, o, g and ?.
The above definition is recursive to arbitrary depth. aaaaai and (ui(nq((y)))s) are both valid type strings, as is a(aa(ui)(qna{ya(yd)})). In order to not hit memory limits, glib.variant.VariantG imposes a limit on recursion depth of 65 nested containers. This is the limit in the D-Bus specification (64) plus one to allow a [gio.dbus_message.DBusMessage] to be nested in a top-level tuple.
The meaning of each of the characters is as follows:
The first type (the basic type) is the key type and the second type is the value type. The reason that the first type is restricted to being a basic type is so that it can easily be hashed.
Any type string of a container that contains an indefinite type is, itself, an indefinite type. For example, the type string a* (corresponding to G_VARIANT_TYPE_ARRAY) is an indefinite type that is a supertype of every array type. (*s) is a supertype of all tuples that contain exactly two items where the second item is a string.
a{?*} is an indefinite type that is a supertype of all arrays containing dictionary entries where the key is any basic type and the value is any type at all. This is, by definition, a dictionary, so this type string corresponds to G_VARIANT_TYPE_DICTIONARY. Note that, due to the restriction that the key of a dictionary entry must be a basic type, {**} is not a valid type string.