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type-checked dictionary templating library for python


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Pystachio is a type-checked dictionary templating library.


Its primary use is for the construction of miniature domain-specific configuration languages. Schemas defined by Pystachio can themselves be serialized and reconstructed into other Python interpreters. Pystachio objects are tailored via Mustache templates, as explained in the section on templating.

Similar projects

This project is unrelated to the defunct Javascript Python interpreter.

Notable related projects:


Tested and works in CPython 2.6.7, 2.7.2, 3.2.1 and PyPy 1.6. Not tested pre-2.6.x and will almost certainly not work. Due to, serialization to/from json will likely break CPython 2.6.1 and earlier because unicode kwargs keys are not supported.


You can define a structured type through the 'Struct' type:

from pystachio import (

class Employee(Struct):
  first = String
  last  = String
  age   = Integer

By default all fields are optional:

>>> Employee().check()

>>> Employee(first = 'brian')
>>> Employee(first = 'brian').check()

But it is possible to make certain fields required:

from pystachio import Required

class Employee(Struct):
  first = Required(String)
  last  = Required(String)
  age   = Integer

We can still instantiate objects with empty fields:

>>> Employee()

But they will fail type checks:

>>> Employee().check()
TypeCheck(FAILED): Employee[last] is required.

Struct objects are purely functional and hence immutable after constructed, however they are composable like functors:

>>> brian = Employee(first = 'brian')
>>> brian(last = 'wickman')
Employee(last=wickman, first=brian)
>>> brian
>>> brian = brian(last='wickman')
>>> brian.check()

Object fields may also acquire defaults:

class Employee(Struct):
  first    = Required(String)
  last     = Required(String)
  age      = Integer
  location = Default(String, "San Francisco")

>>> Employee()
Employee(location=San Francisco)

Schemas wouldn't be terribly useful without the ability to be hierarchical:

class Location(Struct):
  city = String
  state = String
  country = String

class Employee(Struct):
  first    = Required(String)
  last     = Required(String)
  age      = Integer
  location = Default(Location, Location(city = "San Francisco"))

>>> Employee(first="brian", last="wickman")
Employee(last=wickman, location=Location(city=San Francisco), first=brian)

>>> Employee(first="brian", last="wickman").check()

The type system

There are five basic types, two basic container types and then the Struct type.

Basic Types

There are five basic types: String, Integer, Float, Boolean and Enum. The first four behave as expected:

>>> Float(1.0).check()
>>> String("1.0").check()
>>> Integer(1).check()
>>> Boolean(False).check()

They also make a best effort to coerce into the appropriate type:

>>> Float("1.0")
>>> String(1.0)
>>> Integer("1")
>>> Boolean("true")

Though the same gotchas apply as standard coercion in Python:

>>> int("1.0")
ValueError: invalid literal for int() with base 10: '1.0'
>>> Integer("1.0")
pystachio.objects.CoercionError: Cannot coerce '1.0' to Integer

with the exception of Boolean which accepts "false" as falsy.

Enum is a factory that produces new enumeration types:

>>> Enum('Red', 'Green', 'Blue')
<class 'pystachio.typing.Enum_Red_Green_Blue'>
>>> Color = Enum('Red', 'Green', 'Blue')
>>> Color('Red')
>>> Color('Brown')
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/Users/wickman/clients/pystachio/pystachio/", line 208, in __init__
    self.__class__.__name__, ', '.join(self.VALUES)))
ValueError: Enum_Red_Green_Blue only accepts the following values: Red, Green, Blue

Enums can also be constructed using namedtuple syntax to generate more illustrative class names:

>>> Enum('Color', ('Red', 'Green', 'Blue'))
<class 'pystachio.typing.Color'>
>>> Color = Enum('Color', ('Red', 'Green', 'Blue'))
>>> Color('Red')

Container types

There are two container types: the List type and the Map type. Lists are parameterized by the type they contain, and Maps are parameterized from a key type to a value type.


You construct a List by specifying its type (it actually behaves like a metaclass, since it produces a type):

>>> List(String)
<class 'pystachio.container.StringList'>
>>> List(String)([])
>>> List(String)(["a", "b", "c"])
StringList(a, b, c)

They compose like expected:

>>> li = List(Integer)
>>> li
<class 'pystachio.container.IntegerList'>
>>> List(li)
<class 'pystachio.container.IntegerListList'>
>>> List(li)([li([1,"2",3]), li([' 2', '3 ', 4])])
IntegerListList(IntegerList(1, 2, 3), IntegerList(2, 3, 4))

Type checking is done recursively:

>> List(li)([li([1,"2",3]), li([' 2', '3 ', 4])]).check()


You construct a Map by specifying the source and destination types:

>>> ages = Map(String, Integer)({
...   'brian': 30,
...   'ian': 15,
...   'robey': 5000
... })
>>> ages
StringIntegerMap(brian => 28, ian => 15, robey => 5000)
>>> ages.check()

Much like all other types, these types are immutable. The only way to "mutate" would be to create a whole new Map. Technically speaking these types are hashable as well, so you can construct stranger composite types (added indentation for clarity.)

>>> fake_ages = Map(String, Integer)({
...   'brian': 28,
...   'ian': 15,
...   'robey': 5000
... })
>>> real_ages = Map(String, Integer)({
...   'brian': 30,
...   'ian': 21,
...   'robey': 35
... })
>>> believability = Map(Map(String, Integer), Float)({
...   fake_ages: 0.2,
...   real_ages: 0.9
... })
>>> believability
  StringIntegerMap(brian => 28, ian => 15, robey => 5000) => 0.2,
  StringIntegerMap(brian => 30, ian => 21, robey => 35) => 0.9)

Object scopes

Objects have "environments": a set of bound scopes that follow the Object around. Objects are still immutable. The act of binding a variable to an Object just creates a new object with an additional variable scope. You can print the scopes by using the scopes function:

>>> String("hello").scopes()

You can bind variables to that object with the bind function:

>>> String("hello").bind(herp = "derp")

The environment variables of an object do not alter equality, for example:

>>> String("hello") == String("hello")
>>> String("hello").bind(foo = "bar") == String("hello")

The object appears to be the same but it carries that scope around with it:

>>> String("hello").bind(herp = "derp").scopes()
[Environment({Ref(herp): 'derp'})]

Furthermore you can bind multiple times:

>>> String("hello").bind(herp = "derp").bind(herp = "extra derp").scopes()
[Environment({Ref(herp): 'extra derp'}), Environment({Ref(herp): 'derp'})]

You can use keyword arguments, but you can also pass dictionaries directly:

>>> String("hello").bind({"herp": "derp"}).scopes()
[Environment({Ref(herp): 'derp'})]

Think of this as a "mount table" for mounting objects at particular points in a namespace. This namespace is hierarchical:

>>> String("hello").bind(herp = "derp", metaherp = {"a": 1, "b": {"c": 2}}).scopes()
[Environment({Ref(herp): 'derp', Ref(metaherp.b.c): '2', Ref(metaherp.a): '1'})]

In fact, you can bind any Namable object, including List, Map, and Struct types directly:

>>> class Person(Struct)
...   first = String
...   last = String
>>> String("hello").bind(Person(first="brian")).scopes()

The Environment object is simply a mechanism to bind arbitrary strings into a namespace compatible with Namable objects.

Because you can bind multiple times, scopes just form a name-resolution order:

>>> (String("hello").bind(Person(first="brian"), first="john")
                    .bind({'first': "jake"}, Person(first="jane"))).scopes()
 Environment({Ref(first): 'jake'}),
 Environment({Ref(first): 'john'}),

The later a variable is bound, the "higher priority" its name resolution becomes. Binding to an object is to achieve the effect of local overriding. But you can also do a lower-priority "global" bindings via in_scope:

>>> env = Environment(globalvar = "global variable", sharedvar = "global shared variable")
>>> obj = String("hello").bind(localvar = "local variable", sharedvar = "local shared variable")
>>> obj.scopes()
[Environment({Ref(localvar): 'local variable', Ref(sharedvar): 'local shared variable'})]

Now we can bind env directly into obj as if they were local variables using bind:

>>> obj.bind(env).scopes()
[Environment({Ref(globalvar): 'global variable', Ref(sharedvar): 'global shared variable'}),
 Environment({Ref(localvar): 'local variable', Ref(sharedvar): 'local shared variable'})]

Alternatively we can bind env into obj as if they were global variables using in_scope:

>>> obj.in_scope(env).scopes()
[Environment({Ref(localvar): 'local variable', Ref(sharedvar): 'local shared variable'}),
 Environment({Ref(globalvar): 'global variable', Ref(sharedvar): 'global shared variable'})]

You can see the local variables take precedence. The use of scoping will become more obvious when in the context of templating.


Simple templates

As briefly mentioned at the beginning, Mustache templates are first class "language" features. Let's look at the simple case of a String to see how Mustache templates might behave.

>>> String('echo {{hello_message}}')
String(echo {{hello_message}})

OK, seems reasonable enough. Now let's look at the more complicated version of a Float:

>>> Float('not.floaty')
CoercionError: Cannot coerce 'not.floaty' to Float

But if we template it, it behaves differently:

>>> Float('{{not}}.{{floaty}}')

Pystachio understands that by introducing a Mustache template, that we should lazily coerce the Float only once it's fully specified by its environment. For example:

>>> not_floaty = Float('{{not}}.{{floaty}}')
>>> not_floaty.bind({'not': 1})

We've bound a variable into the environment of not_floaty. It's still not floaty:

>>> not_floaty.bind({'not': 1}).check()
TypeCheck(FAILED): u'1.{{floaty}}' not a float

However, once it becomes fully specified, the picture changes:

>>> floaty = not_floaty.bind({'not': 1, 'floaty': 0})
>>> floaty
>>> floaty.check()

Of course, the coercion can only take place if the environment is legit:

>>> not_floaty.bind({'not': 1, 'floaty': 'GARBAGE'})
CoercionError: Cannot coerce '1.GARBAGE' to Float

It's worth noting that floaty has not been coerced permanently:

>>> floaty
>>> floaty.bind({'not': 2})

In fact, floaty continues to store the template; it's just hidden from view and interpolated on-demand:

>>> floaty._value

As we mentioned before, objects have scopes. Let's look at the case of floaty:

>>> floaty = not_floaty.bind({'not': 1, 'floaty': 0})
>>> floaty
>>> floaty.scopes()
[Environment({Ref(not): '1', Ref(floaty): '0'})]

But if we bind not = 2:

>>> floaty.bind({'not': 2})
>>> floaty.bind({'not': 2}).scopes()
[Environment({Ref(not): '2'}), Environment({Ref(floaty): '0', Ref(not): '1'})]

If we had merely just evaluated floaty in the scope of not = 2, it would have behaved differently:

>>> floaty.in_scope({'not': 2})

The interpolation of template variables happens in scope order from top down. Ultimately bind just prepends a scope to the list of scopes and in_scope appends a scope to the end of the list of scopes.

Complex templates

Remember however that you can bind any Namable object, which includes List, Map, Struct and Environment types, and these are hierarchical. Take for example a schema that defines a UNIX process:

class Process(Struct):
  name = Default(String, '{{}}')
  cmdline = String

class ProcessConfig(Struct):
  name = String
  ports = Map(String, Integer)

The expectation could be that Process structures are always interpolated in an environment where config is set to the ProcessConfig.

For example:

>>> webserver = Process(cmdline = "bin/tfe --listen={{config.ports[http]}} --health={{config.ports[health]}}")
>>> webserver
Process(cmdline=bin/tfe --listen={{config.ports[http]}} --health={{config.ports[health]}}, name={{}})

Now let's define its configuration:

>>> app_config = ProcessConfig(name = "tfe", ports = {'http': 80, 'health': 8888})
>>> app_config
ProcessConfig(name=tfe, ports=StringIntegerMap(health => 8888, http => 80))

And let's evaluate the configuration:

>>> webserver % Environment(config = app_config)
Process(cmdline=bin/tfe --listen=80 --health=8888, name=tfe)

The %-based interpolation is just shorthand for in_scope.

List types and Map types are dereferenced as expected in the context of {{}}-style mustache templates, using [index] for List types and [value] for Map types. Struct types are dereferenced using .-notation.

For example, {{[23][baz].bang}} translates to a name lookup chain of foo (Struct) => bar (List or Map) => 23 (Map) => baz (Struct) => bang, ensuring the type consistency at each level of the lookup chain.

Templating scope inheritance

The use of templating is most powerful in the use of Struct types where parent object scope is inherited by all children during interpolation.

Let's look at the example of building a phone book type.

class PhoneBookEntry(Struct):
  name = Required(String)
  number = Required(Integer)

class PhoneBook(Struct):
  city = Required(String)
  people = List(PhoneBookEntry)

>>> sf = PhoneBook(city = "San Francisco").bind(areacode = 415)
>>> sj = PhoneBook(city = "San Jose").bind(areacode = 408)

We met a girl last night in a bar, her name was Jenny, and her number was 8 6 7 5 3 oh nayee-aye-in. But in the bay area, you never know what her area code could be, so we template it:

>>> jenny = PhoneBookEntry(name = "Jenny", number = "{{areacode}}8675309")

But brian is a Nebraskan farm boy from the 402 and took his number with him:

>>> brian = PhoneBookEntry(name = "Brian", number = "{{areacode}}5551234")
>>> brian = brian.bind(areacode = 402)

If we assume that Jenny is from San Francisco, then we look her up in the San Francisco phone book:

>>> sf(people = [jenny])
PhoneBook(city=San Francisco, people=PhoneBookEntryList(PhoneBookEntry(name=Jenny, number=4158675309)))

But it's equally likely that she could be from San Jose:

>>> sj(people = [jenny])
PhoneBook(city=San Jose, people=PhoneBookEntryList(PhoneBookEntry(name=Jenny, number=4088675309)))

If we bind jenny to one of the phone books, she inherits the area code from her parent object. Of course, brian is from Nebraska and he kept his number, so San Jose or San Francisco, his number remains the same:

>>> sf(people = [jenny, brian])
PhoneBook(city=San Francisco,
          people=PhoneBookEntryList(PhoneBookEntry(name=Jenny, number=4158675309),
                                    PhoneBookEntry(name=Brian, number=4025551234)))

Dictionary type-checking

Because of how Struct based schemas are created, the constructor of such a schema behaves like a deserialization mechanism from a straight Python dictionary. In a sense, deserialization comes for free. Take the schema defined below:

class Resources(Struct):
  cpu  = Required(Float)
  ram  = Required(Integer)
  disk = Default(Integer, 2 * 2**30)

class Process(Struct):
  name         = Required(String)
  resources    = Required(Resources)
  cmdline      = String
  max_failures = Default(Integer, 1)

class Task(Struct):
  name         = Required(String)
  processes    = Required(List(Process))
  max_failures = Default(Integer, 1)

Let's write out a task as a dictionary, as we would expect to see from the schema:

task = {
  'name': 'basic',
  'processes': [
      'resources': {
         'cpu': 1.0,
         'ram': 100
      'cmdline': 'echo hello world'

And instantiate it as a Task (indentation provided for clarity):

>>> tsk = Task(task)
>>> tsk
Task(processes=ProcessList(Process(cmdline=echo hello world, max_failures=1,
                                   resources=Resources(disk=2147483648, ram=100, cpu=1.0))),
     max_failures=1, name=basic)

The schema that we defined as a Python class structure is applied to the dictionary. We can use this schema to type-check the dictionary:

>>> tsk.check()
TypeCheck(FAILED): Task[processes] failed: Element in ProcessList failed check: Process[name] is required.

It turns out that we forgot to specify the name of the Process in our process list, and it was a Required field. If we update the dictionary to specify 'name', it will type check successfully.

Type construction

It is possible to serialize constructed types, pickle them and send them around with your dictionary data in order to do portable type checking.


Every type in Pystachio has a serialize_type method which is used to describe the type in a portable way. The basic types are uninteresting:

>>> String.serialize_type()
>>> Integer.serialize_type()
>>> Float.serialize_type()

The notation is simply: String types are produced by the "String" type factory. They are not parameterized types so they need no additional type parameters. However, Lists and Maps are parameterized:

>>> List(String).serialize_type()
('List', ('String',))
>>> Map(Integer,String).serialize_type()
('Map', ('Integer',), ('String',))
>>> Map(Integer,List(String)).serialize_type()
('Map', ('Integer',), ('List', ('String',)))

Furthermore, composite types created with Struct are also serializable. Take the composite types defined in the previous section: Task, Process and Resources.

>>> from pprint import pprint
>>> pprint(Resources.serialize_type(), indent=2, width=100)
( 'Struct',
  ('cpu', (True, (), True, ('Float',))),
  ('disk', (False, 2147483648, False, ('Integer',))),
  ('ram', (True, (), True, ('Integer',))))

In other words, the Struct factory is producing a type with a set of type parameters: Resources is the name of the struct, cpu, disk and ram are attributes of the type.

If you serialize Task, it recursively serializes its children types:

>>> pprint(Task.serialize_type(), indent=2, width=100)
( 'Struct',
  ('max_failures', (False, 1, False, ('Integer',))),
  ('name', (True, (), True, ('String',))),
  ( 'processes',
    ( True,
      ( 'List',
        ( 'Struct',
          ('cmdline', (False, (), True, ('String',))),
          ('max_failures', (False, 1, False, ('Integer',))),
          ('name', (True, (), True, ('String',))),
          ( 'resources',
            ( True,
              ( 'Struct',
                ('cpu', (True, (), True, ('Float',))),
                ('disk', (False, 2147483648, False, ('Integer',))),
                ('ram', (True, (), True, ('Integer',)))))))))))


Given a type tuple produced by serialize_type, you can then use TypeFactory.load from pystachio.typing to load a type into an interpreter. For example:

>>> pprint(TypeFactory.load(Resources.serialize_type()))
{'Float': <class 'pystachio.basic.Float'>,
 'Integer': <class 'pystachio.basic.Integer'>,
 'Resources': <class 'pystachio.typing.Resources'>}

TypeFactory.load returns a map from type name to the fully reified type for all types required to describe the serialized type, including children. In the example of Task above:

>>> pprint(TypeFactory.load(Task.serialize_type()))
{'Float': <class 'pystachio.basic.Float'>,
 'Integer': <class 'pystachio.basic.Integer'>,
 'Process': <class 'pystachio.typing.Process'>,
 'ProcessList': <class 'pystachio.typing.ProcessList'>,
 'Resources': <class 'pystachio.typing.Resources'>,
 'String': <class 'pystachio.basic.String'>,
 'Task': <class 'pystachio.typing.Task'>}

TypeFactory.load also takes an into keyword argument, so you can do TypeFactory.load(type, into=globals()) in order to deposit them into your interpreter:

>>> from pystachio import *
>>> TypeFactory.load(( 'Struct',
...   'Task',
...   ('max_failures', (False, 1, False, ('Integer',))),
...   ('name', (True, (), True, ('String',))),
...   ( 'processes',
...     ( True,
...       (),
...       True,
...       ( 'List',
...         ( 'Struct',
...           'Process',
...           ('cmdline', (False, (), True, ('String',))),
...           ('max_failures', (False, 1, False, ('Integer',))),
...           ('name', (True, (), True, ('String',))),
...           ( 'resources',
...             ( True,
...               (),
...               True,
...               ( 'Struct',
...                 'Resources',
...                 ('cpu', (True, (), True, ('Float',))),
...                 ('disk', (False, 2147483648, False, ('Integer',))),
...                 ('ram', (True, (), True, ('Integer',))))))))))), into=globals())
>>> Task
<class 'pystachio.typing.Task'>
>>> Process
<class 'pystachio.typing.Process'>
>>> Task().check()
TypeCheck(FAILED): Task[processes] is required.
>>> Resources().check()
TypeCheck(FAILED): Resources[ram] is required.
>>> Resources(cpu = 1.0, ram = 1024, disk = 1024).check()


Types produced by TypeFactory.load are reified types but they are not identical to each other. This could be provided in the future via type memoization but that would require keeping some amount of state around.

Instead, __instancecheck__ has been provided, so that you can do isinstance checks:

>>> Task
<class 'pystachio.typing.Task'>
>>> Task =={}, *Task.serialize_type())
>>> isinstance(Task(),{}, *Task.serialize_type()))


@wickman (Brian Wickman)

Thanks to @marius for some of the original design ideas, @benh, @jsirois, @wfarner and others for constructive comments.

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