Introducing to Python slots method

Chris Chen — Tue, 22 Feb 2022 04:05:18 +0000

Just came across this special method __slots__
According to the Document, __slots__ allow us to explicitly declare data members (like properties) and deny the creation of __dict__ and __weakref__ (unless explicitly declared in __slots__ or available in a parent), which means we can use slots to avoid attributes to be dynamically created.

Example

When you create a class, you can access its attributes like this:

class Person:
    def __init__(self,
                 first_name,
                 last_name):
        self.first_name = first_name
        self.last_name = last_name

person = Person('Chris', 'Ryan')
print(person.first_name) # Chris
print(person.last_name) # Chen

Also, you can easily add new attribute:

person.age = 25
print(person.age) # 25

BUT, what if we want to prevent the attributes from being dynamically created?
In this case, I only want my Person class to have first_name and last_name, two attributes.
Try this:

class Person:
    __slots__ = ['first_name', 'last_name']

    def __init__(self,
                 first_name,
                 last_name):
        self.first_name = first_name
        self.last_name = last_name

person = Person('Chris', 'Ryan')
person.age = 25
# AttributeError: 'Person' object has no attribute 'age'

Thanks for reading!

My go-to docker-compose setup for Python development

Chris Chen — Wed, 16 Feb 2022 09:29:50 +0000

I've been using docker-compose instead of venv module for Python development for a while. Here is my go-to setup, please feel free to give me some suggestions and comments!

Folder Structure

Dockefile

# base image
FROM python:3.10

# install packages
RUN apt-get update \
    && apt-get upgrade -y
RUN apt-get install -y \
    vim \
    curl \
    pandoc \
    texlive-xetex \
    texlive-fonts-recommended \
    texlive-plain-generic

# create user and home directory
RUN useradd -m -d /home/devuser -s /bin/bash devuser
USER devuser

ENV PATH=/home/devuser/.local/bin:$PATH

# upgrade pip and install python packages
RUN /usr/local/bin/python -m pip install --upgrade pip
COPY requirements.txt .
RUN pip3 install --no-cache-dir -r requirements.txt

docker-compose.yml

version: '3.8'
services:
  jupyterlab:
    container_name: jupyterlab
    build: .
    ports:
      # for jupyter lab
      - 8888:8888
    volumes:
      - ./src:/home/devuser/src
    working_dir: /home/devuser/src
    command: jupyter lab --port=8888 --ip=0.0.0.0

And then just docker-compose up -d --build, you'll have your docker container up and running!
To get the url or token for jupyter lab, simply run docker logs jupyterlab

This is how I like to setup my Python environment for data processing-related project.

Database Migration with Python

Chris Chen — Tue, 15 Feb 2022 09:34:45 +0000

I think the idea of data migration is similar to git commit. Every migration is a descriptive file that documents changes you've made to the database, such as adding new columns or tables.
Let's dive right into it!

Requirement

alembic documentation



# python version
python==3.10

# requirements.txt
alembic==1.7.6
sqlalchemy==1.4.25
python-dotenv==0.19.0

Folder structure

backend/

alembic init ./app/migration
This command will generate a alembic.ini file and a migration folder

config alembic.ini

Make sure the path is correct



[alembic]
# path to migration scripts
script_location = ./app/migration

You can set up your sqlalchemy url here, but I prefer doing it in env.py



sqlalchemy.url = mysql+pymysql://user:password@host/database

model/

Define your model class in the model folder



# base.py
from sqlalchemy import Column, DateTime, func
from sqlalchemy.dialects.mysql import INTEGER
from sqlalchemy.ext.declarative import declarative_base


Base = declarative_base()


class BaseMixin:
    def __init__(self, **kwargs):
        for key, value in kwargs.items():
            if key in dir(self):
                exec(f"self.{key} = {value}")

    id = Column(INTEGER(unsigned=True), primary_key=True, autoincrement=True)
    created_at = Column(DateTime(timezone=True), server_default=func.now())
    updated_at = Column(
        DateTime(timezone=True), 
        server_default=func.now(), 
        onupdate=func.now())



# model.py
from .base import Base, BaseMixin
from sqlalchemy import Column, String, Integer


class Users(Base, BaseMixin):
    __tablename__ = 'users'

    username = Column(String(50))
    age = Column(Integer())



# __init__.py
from .base import Base, BaseMixin
from .model import Users

config env.py

I make some changes upon the default env.py



from logging.config import fileConfig
from sqlalchemy import create_engine
from alembic import context
from dotenv import load_dotenv

import os
import sys

sys.path.append(os.path.split(os.getcwd())[0])

load_dotenv()

# this is the Alembic Config object, which provides
# access to the values within the .ini file in use.
config = context.config

# Interpret the config file for Python logging.
# This line sets up loggers basically.
fileConfig(config.config_file_name)

# add your model's MetaData object here
# for 'autogenerate' support
from app.model import Base, Users
target_metadata = Base.metadata

# other values from the config, defined by the needs of env.py,
# can be acquired:
# my_important_option = config.get_main_option("my_important_option")
# ... etc.

def get_url():

    return os.getenv('MYSQL_URL')


def run_migrations_offline():
    """Run migrations in 'offline' mode.

    This configures the context with just a URL
    and not an Engine, though an Engine is acceptable
    here as well.  By skipping the Engine creation
    we don't even need a DBAPI to be available.

    Calls to context.execute() here emit the given string to the
    script output.

    """
    url = get_url()
    context.configure(
        url=url,
        target_metadata=target_metadata,
        literal_binds=True,
        dialect_opts={"paramstyle": "named"},
    )

    with context.begin_transaction():
        context.run_migrations()


def run_migrations_online():
    """Run migrations in 'online' mode.

    In this scenario we need to create an Engine
    and associate a connection with the context.

    """
    connectable = create_engine(get_url())

    with connectable.connect() as connection:
        context.configure(
            connection=connection, target_metadata=target_metadata
        )

        with context.begin_transaction():
            context.run_migrations()


if context.is_offline_mode():
    run_migrations_offline()
else:
    run_migrations_online()

Add path to system ```python

import os
import sys

sys.path.append(os.path.split(os.getcwd())[0])

* Load *.env* file
```python


from dotenv import load_dotenv

load_dotenv()

Add your metadata object for --autogenerate ```python

from app.model import Base, Users
target_metadata = Base.metadata

* Read url from *.env*
```python


def get_url():

    return os.getenv('MYSQL_URL')

Modify default function

backend/

First commit

alembic revision --autogenerate -m "first commit"
In /migration/versions folder, you should see a python file like this



"""first commit

Revision ID: 9453de59bdc9

Revises: 

Create Date: 2022-02-15 10:21:24.459889

"""

from alembic import op

import sqlalchemy as sa

from sqlalchemy.dialects import mysql

# revision identifiers, used by Alembic.

revision = '9453de59bdc9'

down_revision = None

branch_labels = None

depends_on = None

def upgrade():

    # ### commands auto generated by Alembic - please adjust! ###

    op.create_table('users',

    sa.Column('id', mysql.INTEGER(unsigned=True), autoincrement=True, nullable=False),

    sa.Column('created_at', sa.DateTime(timezone=True), server_default=sa.text('now()'), nullable=True),

    sa.Column('updated_at', sa.DateTime(timezone=True), server_default=sa.text('now()'), nullable=True),

    sa.Column('username', sa.String(length=50), nullable=True),

    sa.Column('age', sa.Integer(), nullable=True),

    sa.PrimaryKeyConstraint('id')

    )

    # ### end Alembic commands ###



def downgrade():

    # ### commands auto generated by Alembic - please adjust! ###

    op.drop_table('users')

    # ### end Alembic commands ###

First migration

alembic upgrade head
This command will run the upgrade operation, and generate the table in your database

DEV Community: Chris Chen

Introducing to Python slots method

Example

Thanks for reading!

My go-to docker-compose setup for Python development

Folder Structure

Dockefile

docker-compose.yml

Database Migration with Python

Requirement

Folder structure

backend/

config alembic.ini

model/

config env.py

backend/

First commit

First migration

Voila! You've learned alembic!

DEV Community: Chris Chen

Introducing to Python __slots__ method

Example

Thanks for reading!

My go-to docker-compose setup for Python development

Folder Structure

Dockefile

docker-compose.yml

Database Migration with Python

Requirement

Folder structure

backend/

config alembic.ini

model/

config env.py

backend/

First commit

First migration

Voila! You've learned alembic!

Introducing to Python slots method