How could I update a valor in a table using SQLALCHEMY

I am using SQLALCHEMY and I can't update the table, i don't know why it doesn't work.

models.py

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, Date

Base = declarative_base()

class Book(Base):
    __tablename__='books'
    id= Column(Integer, primary_key=True) 
    title=Column(String)
    author = Column(String)
    pages = Column(String)
    published=Column(Date)

    def __repr__(self):
        return "<Book(title='{}', author='{}', pages={}, published={})>"\
            .format(self.title, self.author, self.pages, self.published)

crud.py

from sqlalchemy import create_engine, update
from sqlalchemy.orm import sessionmaker
engine = create_engine(config.DATABASE_URI)
Session = sessionmaker(bind=engine)
s= Session()
def recreate_database():
    models.Base.metadata.drop_all(engine)
    models.Base.metadata.create_all(engine)

I insert the first row on the table 'books' :

book = models.Book(
    title='Deep Learning',
    author='Ian Goodfellow',
    pages=775,
    published=datetime(2016,11,18)
)

using:

recreate_database()
s.add(book)

Then I verify that row is added:

<Book(title='Deep Learning', author='Ian Goodfellow', pages=775, published=2016-11-18 00:00:00)>

And I try to update that valor changing the value of 'pages' to 900 using:

libro=models.Base.metadata.tables['books']
u=update(libro)
u=u.values({"pages": 900})
u=u.where(libro.c.title=="Deep Learning")
engine.execute(u)
s.commit()
s.close()

I don't get error but it does not update the value of pages.

I appreciate if you could tell me where is the error on my code to update the value.

Thanks !!