The document provides an overview and examples of using the Trafaret Python library for data validation and conversion. Some key points: - Trafaret validates data and converts it to the expected type - It allows defining validation rules via simple functions and is easy to extend - Examples show validating dictionaries, lists, integers, strings, and custom data types - Trafaret can be used to validate configuration, API requests/responses, and model properties

1. About me
Mikhail Krivushin
tech lead at AppCraft
from Samara
github: deepwalker
krivushinme@gmail.com
• “Don’t fear” talk author
• Can tie bootlaces

2. Trafaret:
Monads and
Python

3. Trafaret
• one from tens of validation libs, but with
nuance
• trafaret not only validates but converts data
• loves simple functions and very easy to extend

4. Trafaret Base
• DataError, Trafaret base class, trafaret.Call
• Int, Float, String, Any, Null, Bool, Type,
Subclass
• StrBool, Atom, URL, Email
• List, Tuple, Enum
• Dict, Key

5. show me the code
import trafaret as t
name = t.RegexpRaw(‘name=(w+)’) & (lambda match: match.groups()[0])
name(‘name=Joe’) == ‘Joe’
APP_CONFIG = t.Dict({‘kill_them_all’: t.StrBool}) >> json.dumps
APP = t.Dict({
‘app_id’: t.Int,
‘name’: t.String,
‘config’: APP_CONFIG,
})
app = App(**APP(request.args))
db.session.add(app)

6. show me the code
def trafaret_error(meth):
@wraps(meth)
def wrapper(*a, **kw):
try:
return meth(*a, **kw)
except t.DataError as de:
return (
ujson.dumps({‘error': de.as_dict()}),
400,
{'Content-Type': ‘application/json’},
)
return wrapper

7. The beginning
Andrey Vlasovskikh: funcparserlib
https://github.com/vlasovskikh/
funcparserlib
null = n('null') >> const(None)
true = n('true') >> const(True)
false = n('false') >> const(False)
string = toktype('String') >> make_string
value = forward_decl()
member = string + op_(':') + value >> tuple

8. The beginning
Victor Kotseruba: Contract
https://github.com/barbuza/contract
from contract import (
IntC, ListC, DictC, StringC,
ContractValidationError,
)
list_of_ints = ListC[IntC]
foobar = DictC(foo=IntC, bar=StringC)
try:
foobar(data)
except ContractValidationError:
print “AAAAAAaaaaaAA”

9. Simple function
def check_int(value: Any) -> bool:
return (
isinstance(value, int)
or (isinstance(value, str) and value.isdigit())
)
def ensure_int(value: Any) -> Union[int, None]:
if (
isinstance(value, int)
or (isinstance(value, str) and value.isdigit())
):
return int(value)
return None

10. Function evolution
def try_int(value: Any) -> Union[int, DataError]:
if (
isinstance(value, int)
or (isinstance(value, str) and value.isdigit())
):
return int(value)
return DataError(‘Not an int’)
def less_then_500(value: int) -> Union[int, DataError]:
if value < 500:
return value
return DataError(‘value too big’)

11. Go Style go go go
try:
value = check_int(arg)
except t.DataError as data_error:
return str(data_error)
try:
value = less_then_500(value)
except t.DataError as data_error:
return str(data_error)
…

12. Legacy: The Dark Times
When Trafaret was young
class Trafaret:
def __init__(self):
self.converters = []
def append(self, converter):
self.converters.append(converter)
def __rshift__(self, converter):
self.append(converter)
int_less_500 = t.Int()
int_less_500.append(less_then_500)
int_less_500 = t.Int() >> less_then_500

13. Legacy: The Dark Times
When Trafaret was young
def int_less_500_producer():
return t.Int() >> less_then_500
int_less_500_producer() >> but_bigger_then_5

14. Monads?
Are you serious? Why do
you need monads in
python? Do you think this
is Haskell?

15. Python
No, this is not what we
like to do
@do(Maybe)
def with_maybe(first_divisor):
val1 = yield mdiv(2.0, 2.0)
val2 = yield mdiv(3.0, 0.0)
val3 = yield mdiv(val1, val2)
mreturn(val3)

16. Category theory
Monoid in the Endc category.

17. Just a whimsical
compose for a
weird functions
simple_func::simple_type -> simple_type
simple_func1 • simple_func2
weird1::simple_type -> complex_type
weird1 >>= weird2

18. Trafaret type
mypy notation:
TrafaretRes = Union[Any, DataError]
Trafaret = Callable[[Any], TrafaretRes]
in haskell you will name it Either

19. Whimsical Compose
Lets compose weird
trafaret functions
def t_and(t1, t2):
def composed(value):
res = t1(value)
if isinstance(res, DataError):
return res
return t2(res)
return composed
int_less_500 = t_and(try_int, less_then_500)
int_less_500 = t.Int & less_then_500

20. Go Style no no no
int_less_then_500 = t.Int & less_then_500
try:
value = int_less_then_500(arg)
except t.DataError as data_error:
return str(data_error)
…

21. Operations
check = t.Int | t.Bool
check(True) == True
import trafaret as t
check = t.Bool & int
check(True) == 1
check = (t.Int | t.Bool) & int
check(True) == 1
compose aka and
good old `or`
all together now

22. import arrow
def check_date(value):
try:
return arrow.get(value)
except arrow.parser.ParserError:
return t.DataError(‘Bad datetime format’)
import ujson
def check_json(value):
try:
return ujson.loads(value)
except ValueError:
return t.DataError(‘Bad JSON value’)
except TypeError:
return t.DataError(‘Bad JSON value’)
Enlarge your T

23. Dict and Key
• everyone needs to check dicts
• keys can be optional, can have default
• dict can ban extra values or ignore them
• do you want to check only dicts actually?
What about MultiDict?

24. Trafaret Dict Solution
• Key are the item getters and trafaret callers
• Dict collects keys output
• you can implement your own Key like a
function or subclass Key
• keys touches value instance, so be careful with
quantum dicts

25. What Key Type Signature Is?
KeyT = Callable[
[Mapping],
Sequence[
Tuple[
str,
Union[Any, t.DataError],
Sequence[str]
]
]
]
def some_key(name, trafaret):
trafaret = t.ensure_trafaret(trafaret)
def check(value):
if name in value:
yield name, t.catch_error(trafaret, value.get(name)), (name,)
else:
yield name, t.DataError(‘is required’), (name,)
return check

26. Dict Usage
check_request = check_json & t.Dict(some_key(‘count’, t.Int))
check_request(‘{“count”: 5}’) == {‘count’: 5}
check_comma_str = t.String() & (lambda s: s.split(‘,’))
t.Dict({
‘count’: t.Int(),
t.Key(‘UserName’) >> ‘username’: t.String(min_length=5),
‘groups’: check_comma_string & t.List(t.Enum(‘by_device’,
‘by_city’)),
})

27. MultiDict Key
class MultiDictKey(t.Key):
def get_data(self, value):
return value.get_all(self.name)
check_request = t.Dict(
t.Key(‘username’, t.String),
MultiDictKey(‘group’, t.List(check_comma_string)),
)

28. Keys Insanity
def xor_key(first, second, trafaret):
trafaret = t.Trafaret._trafaret(trafaret)
def check_(value):
if (first in value) ^ (second in value):
key = first if first in value else second
yield first, t.catch_error(trafaret, value[key]), (key,)
elif first in value and second in value:
yield first, t.DataError(error=f'correct only if {second} is not defined'), (first,)
yield second, t.DataError(error=f'correct only if {first} is not defined'), (second,)
else:
yield first, t.DataError(error=f'is required if {second} is not defined'), (first,)
yield second, t.DataError(error=f'is required if {first} is not defined'), (second,)
return check_

29. Sugar
from trafaret.constructor import construct, C
construct({
‘a’: int,
‘b’: [bool],
‘c’: (str, str),
})
C & int & bool

30. Form Helpers
fold
unfold
>>> unfold({'a': [1,2], 'b': {'c': 'bla'}})
{'a__0': 1, 'a__1': 2, 'b__c': 'bla'}
>>> fold({'a__0': 1, 'a__1': 2, 'b__c': 'bla'})
{'a': [1, 2], 'b': {'c': 'bla'}}

31. Why not competitors?
Weird DSLs with pain

32. Questions?
@deepwalker/
trafaret
trafaret.rb
pundle
backslant
aldjemy