morphology 0.11.12
# Morphology is a Python data validation library
License: BSD

# Morphology is a Python data validation library

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Morphology is a Python data validation library. It is primarily intended for validating data coming into Python as JSON,
YAML, etc.

It has three goals:

1.  Simplicity.
2.  Support for complex data structures.
3.  Provide useful error messages.

## Contact

To file a bug, create a [new issue](https://github.com/nathanielford/morphology/issues/new) on GitHub with a short example of how to replicate the issue.

## Documentation

The documentation is provided [here](http://nathanielford.github.io/morphology/).

## Changelog

See [CHANGELOG.md](https://github.com/nathanielford/morphology/blob/master/CHANGELOG.md).

## Show me an example

Twitter's [user search API](https://dev.twitter.com/rest/reference/get/users/search) accepts
query URLs like:

```
$ curl 'https://api.twitter.com/1.1/users/search.json?q=python&per_page=20&page=1'
```

To validate this we might use a schema like:

```pycon
>>> from morphology import Schema
>>> schema = Schema({
...   'q': str,
...   'per_page': int,
...   'page': int,
... })

```

This schema very succinctly and roughly describes the data required by
the API, and will work fine. But it has a few problems. Firstly, it
doesn't fully express the constraints of the API. According to the API,
`per_page` should be restricted to at most 20, defaulting to 5, for
example. To describe the semantics of the API more accurately, our
schema will need to be more thoroughly defined:

```pycon
>>> from morphology import Required, All, Length, Range
>>> schema = Schema({
...   Required('q'): All(str, Length(min=1)),
...   Required('per_page', default=5): All(int, Range(min=1, max=20)),
...   'page': All(int, Range(min=0)),
... })

```

This schema fully enforces the interface defined in Twitter's
documentation, and goes a little further for completeness.

"q" is required:

```pycon
>>> from morphology import MultipleInvalid, Invalid
>>> try:
...   schema({})
...   raise AssertionError('MultipleInvalid not raised')
... except MultipleInvalid as e:
...   exc = e
>>> str(exc) == "required key not provided @ data['q']"
True

```

...must be a string:

```pycon
>>> try:
...   schema({'q': 123})
...   raise AssertionError('MultipleInvalid not raised')
... except MultipleInvalid as e:
...   exc = e
>>> str(exc) == "expected str for dictionary value @ data['q']"
True

```

...and must be at least one character in length:

```pycon
>>> try:
...   schema({'q': ''})
...   raise AssertionError('MultipleInvalid not raised')
... except MultipleInvalid as e:
...   exc = e
>>> str(exc) == "length of value must be at least 1 for dictionary value @ data['q']"
True
>>> schema({'q': '#topic'}) == {'q': '#topic', 'per_page': 5}
True

```

"per\_page" is a positive integer no greater than 20:

```pycon
>>> try:
...   schema({'q': '#topic', 'per_page': 900})
...   raise AssertionError('MultipleInvalid not raised')
... except MultipleInvalid as e:
...   exc = e
>>> str(exc) == "value must be at most 20 for dictionary value @ data['per_page']"
True
>>> try:
...   schema({'q': '#topic', 'per_page': -10})
...   raise AssertionError('MultipleInvalid not raised')
... except MultipleInvalid as e:
...   exc = e
>>> str(exc) == "value must be at least 1 for dictionary value @ data['per_page']"
True

```

"page" is an integer \>= 0:

```pycon
>>> try:
...   schema({'q': '#topic', 'per_page': 'one'})
...   raise AssertionError('MultipleInvalid not raised')
... except MultipleInvalid as e:
...   exc = e
>>> str(exc)
"expected int for dictionary value @ data['per_page']"
>>> schema({'q': '#topic', 'page': 1}) == {'q': '#topic', 'page': 1, 'per_page': 5}
True

```

## Defining schemas

Schemas are nested data structures consisting of dictionaries, lists,
scalars and *validators*. Each node in the input schema is pattern
matched against corresponding nodes in the input data.

### Literals

Literals in the schema are matched using normal equality checks:

```pycon
>>> schema = Schema(1)
>>> schema(1)
1
>>> schema = Schema('a string')
>>> schema('a string')
'a string'

```

### Types

Types in the schema are matched by checking if the corresponding value
is an instance of the type:

```pycon
>>> schema = Schema(int)
>>> schema(1)
1
>>> try:
...   schema('one')
...   raise AssertionError('MultipleInvalid not raised')
... except MultipleInvalid as e:
...   exc = e
>>> str(exc) == "expected int"
True

```

### URL's

URL's in the schema are matched by using `urlparse` library.

```pycon
>>> from morphology import Url
>>> schema = Schema(Url())
>>> schema('http://w3.org')
'http://w3.org'
>>> try:
...   schema('one')
...   raise AssertionError('MultipleInvalid not raised')
... except MultipleInvalid as e:
...   exc = e
>>> str(exc) == "expected a URL"
True

```

### Lists

Lists in the schema are treated as a set of valid values. Each element
in the schema list is compared to each value in the input data:

```pycon
>>> schema = Schema([1, 'a', 'string'])
>>> schema([1])
[1]
>>> schema([1, 1, 1])
[1, 1, 1]
>>> schema(['a', 1, 'string', 1, 'string'])
['a', 1, 'string', 1, 'string']

```

However, an empty list (`[]`) is treated as is. If you want to specify a list that can
contain anything, specify it as `list`:

```pycon
>>> schema = Schema([])
>>> try:
...   schema([1])
...   raise AssertionError('MultipleInvalid not raised')
... except MultipleInvalid as e:
...   exc = e
>>> str(exc) == "not a valid value @ data[1]"
True
>>> schema([])
[]
>>> schema = Schema(list)
>>> schema([])
[]
>>> schema([1, 2])
[1, 2]

```

### Validation functions

Validators are simple callables that raise an `Invalid` exception when
they encounter invalid data. The criteria for determining validity is
entirely up to the implementation; it may check that a value is a valid
username with `pwd.getpwnam()`, it may check that a value is of a
specific type, and so on.

The simplest kind of validator is a Python function that raises
ValueError when its argument is invalid. Conveniently, many builtin
Python functions have this property. Here's an example of a date
validator:

```pycon
>>> from datetime import datetime
>>> def Date(fmt='%Y-%m-%d'):
...   return lambda v: datetime.strptime(v, fmt)

```

```pycon
>>> schema = Schema(Date())
>>> schema('2013-03-03')
datetime.datetime(2013, 3, 3, 0, 0)
>>> try:
...   schema('2013-03')
...   raise AssertionError('MultipleInvalid not raised')
... except MultipleInvalid as e:
...   exc = e
>>> str(exc) == "not a valid value"
True

```

In addition to simply determining if a value is valid, validators may
mutate the value into a valid form. An example of this is the
`Coerce(type)` function, which returns a function that coerces its
argument to the given type:

```python
def Coerce(type, msg=None):
    """Coerce a value to a type.

    If the type constructor throws a ValueError, the value will be marked as
    Invalid.
    """
    def f(v):
        try:
            return type(v)
        except ValueError:
            raise Invalid(msg or ('expected %s' % type.__name__))
    return f

```

This example also shows a common idiom where an optional human-readable
message can be provided. This can vastly improve the usefulness of the
resulting error messages.

### Dictionaries

Each key-value pair in a schema dictionary is validated against each
key-value pair in the corresponding data dictionary:

```pycon
>>> schema = Schema({1: 'one', 2: 'two'})
>>> schema({1: 'one'})
{1: 'one'}

```

#### Extra dictionary keys

By default any additional keys in the data, not in the schema will
trigger exceptions:

```pycon
>>> schema = Schema({2: 3})
>>> try:
...   schema({1: 2, 2: 3})
...   raise AssertionError('MultipleInvalid not raised')
... except MultipleInvalid as e:
...   exc = e
>>> str(exc) == "extra keys not allowed @ data[1]"
True

```

This behaviour can be altered on a per-schema basis. To allow
additional keys use
`Schema(..., extra=ALLOW_EXTRA)`:

```pycon
>>> from morphology import ALLOW_EXTRA
>>> schema = Schema({2: 3}, extra=ALLOW_EXTRA)
>>> schema({1: 2, 2: 3})
{1: 2, 2: 3}

```

To remove additional keys use
`Schema(..., extra=REMOVE_EXTRA)`:

```pycon
>>> from morphology import REMOVE_EXTRA
>>> schema = Schema({2: 3}, extra=REMOVE_EXTRA)
>>> schema({1: 2, 2: 3})
{2: 3}

```

It can also be overridden per-dictionary by using the catch-all marker
token `extra` as a key:

```pycon
>>> from morphology import Extra
>>> schema = Schema({1: {Extra: object}})
>>> schema({1: {'foo': 'bar'}})
{1: {'foo': 'bar'}}

```

#### Required dictionary keys

By default, keys in the schema are not required to be in the data:

```pycon
>>> schema = Schema({1: 2, 3: 4})
>>> schema({3: 4})
{3: 4}

```

Similarly to how extra\_ keys work, this behaviour can be overridden
per-schema:

```pycon
>>> schema = Schema({1: 2, 3: 4}, required=True)
>>> try:
...   schema({3: 4})
...   raise AssertionError('MultipleInvalid not raised')
... except MultipleInvalid as e:
...   exc = e
>>> str(exc) == "required key not provided @ data[1]"
True

```

And per-key, with the marker token `Required(key)`:

```pycon
>>> schema = Schema({Required(1): 2, 3: 4})
>>> try:
...   schema({3: 4})
...   raise AssertionError('MultipleInvalid not raised')
... except MultipleInvalid as e:
...   exc = e
>>> str(exc) == "required key not provided @ data[1]"
True
>>> schema({1: 2})
{1: 2}

```

#### Optional dictionary keys

If a schema has `required=True`, keys may be individually marked as
optional using the marker token `Optional(key)`:

```pycon
>>> from morphology import Optional
>>> schema = Schema({1: 2, Optional(3): 4}, required=True)
>>> try:
...   schema({})
...   raise AssertionError('MultipleInvalid not raised')
... except MultipleInvalid as e:
...   exc = e
>>> str(exc) == "required key not provided @ data[1]"
True
>>> schema({1: 2})
{1: 2}
>>> try:
...   schema({1: 2, 4: 5})
...   raise AssertionError('MultipleInvalid not raised')
... except MultipleInvalid as e:
...   exc = e
>>> str(exc) == "extra keys not allowed @ data[4]"
True

```

```pycon
>>> schema({1: 2, 3: 4})
{1: 2, 3: 4}

```

### Recursive / nested schema

You can use `morphology.Self` to define a nested schema:

```pycon
>>> from morphology import Schema, Self
>>> recursive = Schema({"more": Self, "value": int})
>>> recursive({"more": {"value": 42}, "value": 41}) == {'more': {'value': 42}, 'value': 41}
True

```

### Extending an existing Schema

Often it comes handy to have a base `Schema` that is extended with more
requirements. In that case you can use `Schema.extend` to create a new
`Schema`:

```pycon
>>> from morphology import Schema
>>> person = Schema({'name': str})
>>> person_with_age = person.extend({'age': int})
>>> sorted(list(person_with_age.schema.keys()))
['age', 'name']

```

The original `Schema` remains unchanged.

### Objects

Each key-value pair in a schema dictionary is validated against each
attribute-value pair in the corresponding object:

```pycon
>>> from morphology import Object
>>> class Structure(object):
...     def __init__(self, q=None):
...         self.q = q
...     def __repr__(self):
...         return '<Structure(q={0.q!r})>'.format(self)
...
>>> schema = Schema(Object({'q': 'one'}, cls=Structure))
>>> schema(Structure(q='one'))
<Structure(q='one')>

```

### Allow None values

To allow value to be None as well, use Any:

```pycon
>>> from morphology import Any

>>> schema = Schema(Any(None, int))
>>> schema(None)
>>> schema(5)
5

```

## Error reporting

Validators must throw an `Invalid` exception if invalid data is passed
to them. All other exceptions are treated as errors in the validator and
will not be caught.

Each `Invalid` exception has an associated `path` attribute representing
the path in the data structure to our currently validating value, as well
as an `error_message` attribute that contains the message of the original
exception. This is especially useful when you want to catch `Invalid`
exceptions and give some feedback to the user, for instance in the context of
an HTTP API.


```pycon
>>> def validate_email(email):
...     """Validate email."""
...     if not "@" in email:
...         raise Invalid("This email is invalid.")
...     return email
>>> schema = Schema({"email": validate_email})
>>> exc = None
>>> try:
...     schema({"email": "whatever"})
... except MultipleInvalid as e:
...     exc = e
>>> str(exc)
"This email is invalid. for dictionary value @ data['email']"
>>> exc.path
['email']
>>> exc.msg
'This email is invalid.'
>>> exc.error_message
'This email is invalid.'

```

The `path` attribute is used during error reporting, but also during matching
to determine whether an error should be reported to the user or if the next
match should be attempted. This is determined by comparing the depth of the
path where the check is, to the depth of the path where the error occurred. If
the error is more than one level deeper, it is reported.

The upshot of this is that *matching is depth-first and fail-fast*.

To illustrate this, here is an example schema:

```pycon
>>> schema = Schema([[2, 3], 6])

```

Each value in the top-level list is matched depth-first in-order. Given
input data of `[[6]]`, the inner list will match the first element of
the schema, but the literal `6` will not match any of the elements of
that list. This error will be reported back to the user immediately. No
backtracking is attempted:

```pycon
>>> try:
...   schema([[6]])
...   raise AssertionError('MultipleInvalid not raised')
... except MultipleInvalid as e:
...   exc = e
>>> str(exc) == "not a valid value @ data[0][0]"
True

```

If we pass the data `[6]`, the `6` is not a list type and so will not
recurse into the first element of the schema. Matching will continue on
to the second element in the schema, and succeed:

```pycon
>>> schema([6])
[6]

```

## Running tests.

Morphology is using nosetests:

    $ nosetests


## Why use Morphology over another validation library?

**Validators are simple callables**
:   No need to subclass anything, just use a function.

**Errors are simple exceptions.**
:   A validator can just `raise Invalid(msg)` and expect the user to get
useful messages.

**Schemas are basic Python data structures.**
:   Should your data be a dictionary of integer keys to strings?
`{int: str}` does what you expect. List of integers, floats or
strings? `[int, float, str]`.

**Designed from the ground up for validating more than just forms.**
:   Nested data structures are treated in the same way as any other
type. Need a list of dictionaries? `[{}]`

**Consistency.**
:   Types in the schema are checked as types. Values are compared as
values. Callables are called to validate. Simple.

## Lineage

Morphology is an almost-direct branch of [this library](https://github.com/alecthomas/voluptuous). 
[This issue](https://github.com/alecthomas/voluptuous/issues/287) was opened, addressing the inappropriate 
nature of the name, but was summarily closed by the original author. Sadly, this prevents an otherwise great library 
from being utilized in professional, inclusive settings, and the only solution was to fork it to address this specific 
issue. It is important to recognize that Alec Thomas and the other contributers there should receive all the credit for 
the functionality here.   

In the future I intend to port any significant upgrades over, and will attempt to keep versions in sync such that one is
interchangeable with the other with a simple replace-all. For various build mishagus reasons, morphology minor versions
will be equivalent to `10x(parent minor version)+c`, where `c` is 0-9 and just has to do with integration fixes.