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Data specification and normalization toolkit

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Data Spec

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What are Specs?

Specs are declarative data specifications written in pure Python code. Specs can be created using the Spec utility function s. Specs provide two useful and related functions. The first is to evaluate whether an arbitrary data structure satisfies the specification. The second function is to conform (or normalize) valid data structures into a canonical format.

The simplest Specs are based on common predicate functions, such as lambda x: isinstance(x, str) which asks "Is the object x an instance of str?". Fortunately, Specs are not limited to being created from single predicates. Specs can also be created from groups of predicates, composed in a variety of useful ways, and even defined for complex data structures. Because Specs are ultimately backed by pure Python code, any question that you can answer about your data in code can be encoded in a Spec.

How to Use

To begin using the spec library, you can simply import the s object:

from dataspec import s

Nearly all of the useful functionality in spec is packed into s.

Spec API

s is a generic Spec constructor, which can be called to generate new Specs from a variety of sources:

  • Enumeration specs:
    • Using a Python set or frozenset: s({"a", "b", ...}), or
    • Using a Python Enum like State, s(State).
  • Collection specs:
    • Using a Python list: s([State])
  • Mapping type specs:
    • Using a Python dict: s({"name": s.is_str})
  • Tuple type specs:
    • Using a Python tuple: s((s.is_str, s.is_num))
  • Specs based on:
    • Using a standard Python predicate: s(lambda x: x > 0)
    • Using a Python function yielding ErrorDetails

Specs are designed to be composed, so each of the above spec types can serve as the base for more complex data definitions. For collection, mapping, and tuple type Specs, Specs will be recursively created for child elements if they are types understood by s.

Specs may also optionally be created with "tags", which are just string names provided in ErrorDetails objects emitted by Spec instance validate methods. Specs are required to have tags and all builtin Spec factories will supply a default tag if one is not given.

The s API also includes several Spec factories for common Python types such as bool, bytes, date, datetime (via s.inst), float (via s.num), int (via s.num), str, time, and uuid.

s also includes several pre-built Specs for basic types which are useful if you only want to verify that a value is of a specific type. All the pre-built Specs are supplied as s.is_{type} on s.

All Specs provide the following API:

  • Spec.is_valid(x) returns a bool indicating if x is valid according to the Spec definition
  • Spec.validate(x) yields consecutive ErrorDetails describing every spec violation for x. By definition, if next(Spec.validate(x)) returns an empty generator, then x satisfies the Spec.
  • Spec.validate_ex(x) throws a ValidationError containing the full list of ErrorDetails of errors occurred validating x if any errors are encountered. Otherwise, returns None.
  • Spec.conform(x) attempts to conform x according to the Spec conformer iff x is valid according to the Spec. Otherwise returns INVALID.
  • Spec.conform_valid(x) conforms x using the Spec conformer, without checking first if x is valid. Useful if you wish to check your data for validity and conform it in separate steps without incurring validation costs twice.
  • Spec.with_conformer(c) returns a new Spec instance with the Conformer c. The old Spec instance is not modified.
  • Spec.with_tag(t) returns a new Spec instance with the Tag t. The old Spec instance is not modified.

Scalar Specs

The simplest data specs are those which evaluate Python's builtin scalar types: strings, integers, floats, and booleans.

You can create a spec which validates strings with s.str(). Common string validations can be specified as keyword arguments, such as the min/max length or a matching regex. If you are only interested in validating that a value is a string without any further validations, spec features the predefined spec s.is_str (note no function call required).

Likewise, numeric specs can be created using s.num(), with several builtin validations available as keyword arguments such as min/max value and narrowing down the specific numeric types. If you are only interested in validating that a value is numeric, you can use the builtin s.is_num or s.is_int or s.is_float specs.

Predicate Specs

You can define a spec using any simple predicate you may have by passing the predicate directly to the s function, since not every valid state of your data can be specified using existing specs.

spec = s(lambda id_: uuid.UUID(id_).version == 4)
spec.is_valid("4716df50-0aa0-4b7d-98a4-1f2b2bcb1c6b")  # True
spec.is_valid("b4e9735a-ee8c-11e9-8708-4c327592fea9")  # False

UUID Specs

In the previous section, we used a simple predicate to check that a UUID was a certain version of an RFC 4122 variant UUID. However, spec includes builtin UUID specs which can simplify the logic here:

spec = s.uuid(versions={4})
spec.is_valid("4716df50-0aa0-4b7d-98a4-1f2b2bcb1c6b")  # True
spec.is_valid("b4e9735a-ee8c-11e9-8708-4c327592fea9")  # False

Additionally, if you are only interested in validating that a value is a UUID, the builting spec s.is_uuid is available.

Date Specs

spec includes some builtin Specs for Python's datetime, date, and time classes. With the builtin specs, you can validate that any of these three class types are before or after a given. Suppose you want to verify that someone is 18 by checking their date of birth:

spec = - timedelta(years=18))
spec.is_valid( - timedelta(years=21))  # True
spec.is_valid( - timedelta(years=12))  # False

For datetimes (instants) and times, you can also use is_aware=True to specify that the instance be timezone-aware (e.g. not naive).

You can use the builtins s.is_date, s.is_inst, and s.is_time if you only want to validate that a value is an instance of any of those classes.

Set (Enum) Specs

Commonly, you may be interested in validating that a value is one of a constrained set of known values. In Python code, you would use an Enum type to model these values. To define an enumermation spec, you can use either pass an existing Enum value into your spec:

class YesNo(Enum):
    YES = "Yes"
    NO = "No"

s(YesNo).is_valid("Yes")    # True
s(YesNo).is_valid("Maybe")  # False

Any valid representation of the Enum value would satisfy the spec, including the value, alias, and actual Enum value (like YesNo.NO).

Additionally, for simpler cases you can specify an enum using Python sets (or frozensets):

s({"Yes", "No"}).is_valid("Yes")    # True
s({"Yes", "No"}).is_valid("Maybe")  # False

Collection Specs

Specs can be defined for values in homogenous collections as well. Define a spec for a homogenous collection as a list passed to s with the first element as the Spec for collection elements:

s([s.num(min_=0)]).is_valid([1, 2, 3, 4])  # True
s([s.num(min_=0)]).is_valid([-11, 2, 3])   # False

You may also want to assert certain conditions that apply to the collection as a whole. Spec allows you to specify an optional dictionary as the second element of the list with a few possible rules applying to the collection as a whole, such as length and collection type.

s([s.num(min_=0), {"kind": list}]).is_valid([1, 2, 3, 4])  # True
s([s.num(min_=0), {"kind": list}]).is_valid({1, 2, 3, 4})  # False

Collection specs conform input collections by applying the element conformer(s) to each element of the input collection. Callers can specify an "into" key in the collection options dictionary as part of the spec to specify which type of collection is emitted by the collection spec default conformer. Collection specs which do not specify the "into" collection type will conform collections into the same type as the input collection.

Tuple Specs

Specs can be defined for heterogenous collections of elements, which is often the use case for Python's tuple type. To define a spec for a tuple, pass a tuple of specs for each element in the collection at the corresponding tuple index:

        s.str("id", format_="uuid"),
        s.str("date_of_birth", format_="iso-date"),
        s("gender", {"M", "F"}),

Tuple specs conform input tuples by applying each field's conformer(s) to the fields of the input tuple to return a new tuple. If each field in the tuple spec has a unique tag and the tuple has a custom tag specified, the default conformer will yield a namedtuple with the tuple spec tag as the type name and the field spec tags as each field name. The type name and field names will be munged to be valid Python identifiers.

Mapping Specs

Specs can be defined for mapping/associative types and objects. To define a spec for a mapping type, pass a dictionary of specs to s. The keys should be the expected key value (most often a string) and the value should be the spec for values located in that key. If a mapping spec contains a key, the spec considers that key required. To specify an optional key in the spec, wrap the key in s.opt. Optional keys will be validated if they are present, but allow the map to exclude those keys without being considered invalid.

        "id": s.str("id", format_="uuid"),
        "first_name": s.str("first_name"),
        "last_name": s.str("last_name"),
        "date_of_birth": s.str("date_of_birth", format_="iso-date"),
        "gender": s("gender", {"M", "F"}),
        s.opt("state"): s("state", {"CA", "GA", "NY"}),

Above the key "state" is optional in tested values, but if it is provided it must be one of "CA", "GA", or "NY".

Note: Mapping specs do not validate that input values only contain the expected set of keys. Extra keys will be ignored. This is intentional behavior.

Mapping specs conform input dictionaries by applying each field's conformer(s) to the fields of the input map to return a new dictionary. As a consequence, the value returned by the mapping spec default conformer will not include any extra keys included in the input. Optional keys will be included in the conformed value if they appear in the input map.

Combination Specs

In most of the previous examples, we used basic builtin Specs. However, real world data often more nuanced specifications for data. Fortunately, Specs were designed to be composed. In particular, Specs can be composed using standard boolean logic. To specify an or spec, you can use s.any(...) with any n specs.

spec = s.any(s.str(format_="uuid"), s.str(maxlength=0))
spec.is_valid("4716df50-0aa0-4b7d-98a4-1f2b2bcb1c6b")  # True
spec.is_valid("")            # True
spec.is_valid("3837273723")  # False

Similarly, to specify an and spec, you can use s.all(...) with any n specs:

spec = s.all(s.str(format_="uuid"), s(lambda id_: uuid.UUID(id_).version == 4))
spec.is_valid("4716df50-0aa0-4b7d-98a4-1f2b2bcb1c6b")  # True
spec.is_valid("b4e9735a-ee8c-11e9-8708-4c327592fea9")  # False

and Specs apply each child Spec's conformer to the value during validation, so you may assume the output of the previous Spec's conformer in subsequent Specs.


Suppose you'd like to define a Spec for validating that a string is at least 10 characters long (ignore encoding nuances), you could define that as follows:

spec = s.str(minlength=10)
spec.is_valid("a string")         # False
spec.is_valid("London, England")  # True

Or perhaps you'd like to check that every number in a list is above a certain value:

spec = s([s.num(min_=70), {"kind": list}])
spec.is_valid([70, 83, 92, 99])  # True
spec.is_valid({70, 83, 92, 99})  # False, as the input collection is a set
spec.is_valid([43, 66, 80, 93])  # False, not all numbers above 70

A more realistic case for a Spec is validating incoming data at the application boundaries. Suppose you're accepting a user profile submission as a JSON object over an HTTP endpoint, you could validate the data like so:

spec = s(
        "id": s.str("id", format_="uuid"),
        "first_name": s.str("first_name"),
        "last_name": s.str("last_name"),
        "date_of_birth": s.str("date_of_birth", format_="iso-date"),
        "gender": s("gender", {"M", "F"}),
        s.opt("state"): s.str(minlength=2, maxlength=2),
spec.is_valid(  # True
        "id": "e1bc9fb2-a4d3-4683-bfef-3acc61b0edcc",
        "first_name": "Carl",
        "last_name": "Sagan",
        "date_of_birth": "1996-12-20",
        "gender": "M",
        "state": "CA",
spec.is_valid(  # True; note that extra keys _are ignored_
        "id": "958e2f55-5fdf-4b84-a522-a0765299ba4b",
        "first_name": "Marie",
        "last_name": "Curie",
        "date_of_birth": "1867-11-07",
        "gender": "F",
        "occupation": "Chemist",
spec.is_valid(  # False; missing "gender" key
        "id": "958e2f55-5fdf-4b84-a522-a0765299ba4b",
        "first_name": "Marie",
        "last_name": "Curie",
        "date_of_birth": "1867-11-07",



Predicates are functions of one argument which return a boolean. Predicates answer questions such as "is x an instance of str?" or "is n greater than 0?". Frequently in Python, predicates are simply expressions used in an if statement. In functional programming languages (and particularly in Lisps), it is more common to encode these predicates in functions which can be combined using lambdas or partials to be reused. Spec encourages that functional paradigm and benefits directly from it.

Predicate functions should satisfy the PredicateFn type and can be wrapped in the PredicateSpec spec type.


Validators are like predicates in that they answer the same fundamental questions about data that predicates do. However, Validators are a Spec concept that allow us to retrieve richer error data from Spec failures than we can natively with a simple predicate. Validators are functions of one argument which return 0 or more ErrorDetails instances (typically yielded as a generator) describing the error.

Validator functions should satisfy the ValidatorFn type and can be wrapped in the ValidatorSpec spec type.


Conformers are functions of one argument, x, that return either a conformed value, which may be x itself, a new value based on x, or the special Spec value INVALID if the value cannot be conformed.

All specs may include conformers. Scalar spec types such as PredicateSpec and ValidatorSpec simply return their argument if it satisfies the spec. Specs for more complex data structures supply a default conformer which produce new data structures after applying any child conformation functions to the data structure elements.


All Specs can be created with optional tags, specified as a string in the first positional argument of any spec creation function. Tags are useful for providing useful names for specs in debugging and validation messages.


  • in dict specs, default child spec tag from corresponding dictionary key
  • break out conformers into separate object? main value would be to propogate .conform_valid() calls all the way through; currently they don't propogate past collection, dict, and tuple specs


MIT License

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