[Sqlalchemy-tickets] Issue #3771: PyMySQL custom converters reset to default / not used during a commit (zzzeek/sqlalchemy)

[Alexandre A. <iss...@bi...>]

New issue 3771: PyMySQL custom converters reset to default / not used during a commit
https://bitbucket.org/zzzeek/sqlalchemy/issues/3771/pymysql-custom-converters-reset-to-default

Alexandre Avanian:

When creating a mysql engine with pymysql and custom converters, the converters dict passed in parameter is taken in account in the pymysql connection object but during a commit, it seems encoders revert to the default ones. See below some sample code.

The error "AttributeError: 'Timestamp' object has no attribute 'translate'" occurs when pymysql cannot find a converter for the type of the value passed.
Dropping in debugger confirms that the encoders are the default ones and the value is of type pandas.Timestamp.

    sqlalchemy: 1.0.13
    pymysql: 0.7.6.None

```
from datetime import datetime
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, DateTime
from pymysql.converters import conversions, escape_datetime
from pandas import Timestamp


Base = declarative_base()
class TestConverter(Base):
    __tablename__ = 'test_converter'

    id = Column(Integer, primary_key=True)
    when = Column(DateTime)

assert escape_datetime(Timestamp('2016-08-12 08:45:00')) == escape_datetime(datetime(2016, 8, 12, 8, 45))
conversions[Timestamp] = escape_datetime
engine = create_engine('mysql+pymysql://user:password@host:port/db', connect_args={'conv': conversions})
Session = sessionmaker(bind=engine)
session = Session()

Base.metadata.create_all(engine)
print(engine.contextual_connect()connection.connection.encoders)  # shows that the encoder for pandas.Timestamp is there

new = TestConverter(when=Timestamp('2016-08-12 08:45:00'))
session.add(new)
session.commit()  # fails with AttributeError: 'Timestamp' object has no attribute 'translate'
```