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Remove number validation on deserialization for DynamoDB resources #4699

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Remove number validation on deserialization for DynamoDB resources #4699

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bbd0320
Fix DynamoDB number deserialization for large numbers with trailing z…
jonathan343 Jan 14, 2026
1c8acb1
Fix DynamoDB trailing-zero number handling in serializer
jonathan343 Mar 20, 2026
d27d275
add comment for DYNAMODB_CONTEXT
jonathan343 Mar 20, 2026
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11 changes: 6 additions & 5 deletions boto3/dynamodb/types.py
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Original file line number Diff line number Diff line change
Expand Up @@ -33,7 +33,8 @@
MAP = 'M'
LIST = 'L'


# Retained for backwards compatibility; serializer/deserializer no longer use
# this context for number handling.
DYNAMODB_CONTEXT = Context(
Emin=-128,
Emax=126,
Expand Down Expand Up @@ -211,10 +212,10 @@ def _serialize_bool(self, value):
return value

def _serialize_n(self, value):
number = str(DYNAMODB_CONTEXT.create_decimal(value))
if number in ['Infinity', 'NaN']:
decimal_value = Decimal(value)
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@SamRemis SamRemis Mar 20, 2026

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This is less familiar code for me, so I'm still trying to reason about all of this.

One alternate solution here could just be removing the rounded trap. Here's the test code I wrote to check it will work:

import decimal
import boto3
dynamodb = boto3.resource('dynamodb')
table = dynamodb.Table('table-name')

print(table.put_item(Item={
    'primary_key': "test",
    #This breaks with the `decimal.Rounded` trap, but is fine with the `decimal.Inexact trap`,
    # the difference being that inexact will allow "rounding" if it's not losing precision.
    # Adding a 1 on the end would break both as expected
    'precise_number': decimal.Decimal("0.12345678901234567890123456789012345678000000")
}))

print(table.get_item(Key={"primary_key": "test"}))

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@SamRemis SamRemis Mar 21, 2026

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This also shifts where an exception happens, so a customer relying on us to raise any of the former exceptions (such as if the number was too big for DDB to except). For a hamfisted example:

import decimal
import boto3

dynamodb = boto3.resource('dynamodb')
table = dynamodb.Table('test-table')
biiiiig = "1e500"

try:
    print(table.put_item(Item={
        'primary_key': "test_biiiiig",
        'precise_number': decimal.Decimal(biiiiig)
    }))
except decimal.Overflow as e:
    print("Hi end user, we are sorry but that number is too big for us to store in our database")
    raise


# Before, we would never reach this code path, but now we will start getting a client error.
#  That's not caught above.  
response = table.get_item(Key={"primary_key": "test_biiiiig"})
print("Your number is: " + str(response.get('Item').get('precise_number')))

Outside of this, I think this is a safe change, but I prefer not to break customer's try/except blocks when we can avoid it. What do you think?

if decimal_value.is_nan() or decimal_value.is_infinite():
raise TypeError('Infinity and NaN not supported')
return number
return str(decimal_value)

def _serialize_s(self, value):
return value
Expand Down Expand Up @@ -286,7 +287,7 @@ def _deserialize_bool(self, value):
return value

def _deserialize_n(self, value):
return DYNAMODB_CONTEXT.create_decimal(value)
return Decimal(value)

def _deserialize_s(self, value):
return value
Expand Down
32 changes: 32 additions & 0 deletions tests/unit/dynamodb/test_types.py
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Original file line number Diff line number Diff line change
Expand Up @@ -85,6 +85,10 @@ def test_serialize_NaN_error(self):
with pytest.raises(TypeError, match=r'Infinity and NaN not supported'):
self.serializer.serialize(Decimal('NaN'))

def test_serialize_infinity_error(self):
with pytest.raises(TypeError, match=r'Infinity and NaN not supported'):
self.serializer.serialize(Decimal('-Infinity'))

def test_serialize_string(self):
assert self.serializer.serialize('foo') == {'S': 'foo'}

Expand Down Expand Up @@ -143,6 +147,21 @@ def test_serialize_map(self):
'M': {'foo': {'S': 'bar'}, 'baz': {'M': {'biz': {'N': '1'}}}}
}

def test_serialize_large_number_with_trailing_zeros(self):
assert self.serializer.serialize(
1234567895171680000000000000000000000000
) == {'N': '1234567895171680000000000000000000000000'}

def test_serialize_large_decimal_with_trailing_zeros(self):
assert self.serializer.serialize(
Decimal('1234567895171680000000000000000000000000')
) == {'N': '1234567895171680000000000000000000000000'}

def test_serialize_large_decimal_defers_validation_to_dynamodb(self):
assert self.serializer.serialize(Decimal('9' * 39)) == {
'N': '999999999999999999999999999999999999999'
}


class TestDeserializer(unittest.TestCase):
def setUp(self):
Expand Down Expand Up @@ -206,3 +225,16 @@ def test_deserialize_map(self):
}
}
) == {'foo': 'mystring', 'bar': {'baz': Decimal('1')}}

def test_deserialize_large_number_with_trailing_zeros(self):
# Numbers with trailing zeros that exceed 38 total digits but have
# <=38 significant digits are valid in DynamoDB and must deserialize.
large_num = '1234567895171680000000000000000000000000'
result = self.deserializer.deserialize({'N': large_num})
assert result == Decimal(large_num)
assert str(result) == large_num

def test_deserialize_and_serialize_large_number_with_trailing_zeros(self):
large_num = '1234567895171680000000000000000000000000'
result = self.deserializer.deserialize({'N': large_num})
assert TypeSerializer().serialize(result) == {'N': large_num}
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