py-app-properties 1.3.0

Java application properties for Python

Python Application Properties

What is this

It is a simple library to inject non-sensitive configurations into class variables. Basically, it's like BaseSettings in pydantic library but for constants in json and yaml formats. py-app-properties can work with different Python types (like tuple, datetime, dataclass and so on) and recursively cast config values to them.

When to use

• If you deal with constants in your code, like error messages, default values for something, numeric coefficients, and so on.
• If you hate global variables, and you like non-python files to store static information.
• If you want to have an easy way to manage different constants depending on environments (like test, dev, prod).
• If you like type hints and clean code.

How to install

To install this library just enter:

pip install py-app-properties

How to use

For injecting values you need only the decorator properties under a target class. By default, the library will search a config file application.yml in the same directory where your file with the used decorator is located, like below:

project_root
|---services
|   |   email_message_service.py
|   |   application.yml
|.....

Example:

services/application.yml:

default_text_style:
  size: 14
  weight: bold
  font: "Times New Roman"
  color:
    - 128
    - 128
    - 128
language_greetings:
  - language: english
    text: hello
  - language: german
    text: hallo
  - language: french
    text: bonjour
wellcome_message: "{greeting}! Thank you for registration, {username}!"
mailing_frequency:
  days: 5
  hours: 12

services/email_message_service.py:

from typing import TypedDict
from dataclasses import dataclass
from datetime import timedelta
from app_properties import properties

@dataclass
class TextStyle:
    size: int
    weight: str
    font: str
    color: tuple[int, int, int] | str

class GreetingDict(TypedDict):
    language: str
    text: str

@properties
class EmailMessageService:
    default_text_style: TextStyle
    language_greetings: list[GreetingDict]
    wellcome_message: str
    mailing_frequency: timedelta | None
    
    # And using these class variables in some methods...

And that's how will look an equivalent of the code above but with "hard-coded" constants, without config files and @properties decorator:

class EmailMessageService:
    default_text_style = TextStyle(
        size=14, weight="bold", font="Times New Roman", color=(128, 128, 128)
    )
    language_greetings = [
      GreetingDict(language="english", text="hello"),
      GreetingDict(language="german", text="hallo"),
      GreetingDict(language="french", text="bonjour"),
    ]
    wellcome_message = "{greeting}! Thank you for registration, {username}!"
    mailing_frequency = timedelta(days=5, hours=12)
    
    # And using these class variables in some methods...

All config values will be inserted and cast according to the type annotations once during the application or script start. Additionally, the decorator takes such params:

• filename - the name of a file with config. By default, it is application.yml. Use a relative path with ../ to read the file from a parent directory;
• type_cast - used to know whether you want to cast config values to the field type. By default, it's True, which means values in a config file will be cast according to the type hints. All types specified in the section supported types will be available for type casting. Also, nested types will be recursively cast. If False, type hinting is ignored, and available types are limited by a file format;
• override_default - used to know whether you want to override the default values of class variables. By default, it is False;
• lazy_init - used to know whether you want to set config values immediately on the application start-up or on demand ("lazily") after calling the method init_props(). By default, it is False;
• root - root key in the config. It's the way to create "namespaces" when you work with multiple classes but use a single config file. It could be a nested value with separation by dots, for example:

# example.yml
services:
  email_service:
    key: some value
  auth_service:
    key: another value

clients:
  translation_client:
    key: value

# and so on...

from app_properties import properties

@properties(filename="example.yml", root="services.email_service")
class EmailService:
    key: str  # will store "some value"


@properties(filename="example.yml", root="services.auth_service")
class AuthService:
    key: str  # will store "another value"

Different environments

Using this library it's easy to manage different environments and corresponding config files. It could be done like so:

import os
from app_properties import properties


@properties(filename=os.getenv("CONFIG_FILENAME", "application.yml"))
class SomeEnvDependingService:
    env_depend_var: str

In this case, you set CONFIG_FILENAME=application-dev.yml in env variables, and py-app-properties will use that file.

Lazy initialization

If you want to create some dataclass instance with filled required data during init, and then populated with config values, you can use the parameter lazy_init for this purpose. All file constants will be injected after calling the method init_props:

# All definitions like in previous examples

@properties(lazy_init=True)
@dataclass
class EmailMessageServiceConfig:
    default_text_style: TextStyle
    language_greetings: list[GreetingDict]
    mailing_frequency: timedelta | None = None
    wellcome_message: str = "some_default_message"

email_config = EmailMessageServiceConfig(
    default_text_style=TextStyle(
        size=16, weight="normal", font="Arial", color="black"
    ),
    language_greetings=[GreetingDict(language="english", text="hello")]
)
# it works like a normal dataclass instance
assert email_config.default_text_style == TextStyle(
    size=16, weight="normal", font="Arial", color="black"
)
assert email_config.mailing_frequency is None
assert email_config.wellcome_message == "some_default_message"

# after calling `init_props`, config values will be injected. 
# It also overrides all values that we set during initialize before.
email_config.init_props()
assert email_config.default_text_style == TextStyle(
    size=14, weight="bold", font="Times New Roman", color=(128, 128, 128)
)
assert email_config.mailing_frequency == timedelta(days=5, hours=12)
assert email_config.wellcome_message == (
    "{greeting}! Thank you for registration, {username}!"
)

Because there are 3 sources of data (default values, values passed during initialization, and config file values), it could be hard to understand how we can resolve this conflict. Bellow is the table to clarify the behavior of the init_props method.

init	default	config	will be used
-	+	-	default
-	+	+	config
+	~	-	init
+	~	+	init \ config
+- provided; -- missing; ~- not affect.

How you can see, when both init and config values provided, they are equally important, but, by default, config have higher priority and overrides init. If you, for some reason, don't want to override already initialized values, only defaults, it's also possible with init_props(override_init=False)

Supported types

The table below shows how config values (json syntax example) are cast to Python types:

Python type	Config file type	Config example
int	int str	10 "10"
float	float int str	10.5 10 "10.5"
str	str	"string value"
bool	bool int str	true / false 1 / 0 "True" / "False", "true" / "false"
None	null	null
dict	dict	{"key": "value"}
list tuple set frozenset	list	["val1", "val2"]
TypedDict	dict	{"str_var": "value"}
NamedTuple	list dict	["value", 10] {"str_val": "value", "int_val": 10}
dataclass	dict	{"str_val": "str", "int_val": 10}
datetime.datetime	str int list dict	"2022-12-11T10:20:23" 1670754600 [2022, 12, 11, 10, 20, 23] {"year": 2022, "month": 12, "day": 11, "hour": 10, "minute": 20, "second": 23}
datetime.date	str list dict	"2022-12-11" [2022, 12, 11] {"year": 2022, "month": 12, "day": 11}
datetime.time	str list dict	"12:30:02" [12, 30, 2] {"hour": 12, "minute": 30, "second": 2}
datetime.timedelta	dict	{"days": 1, "hours": 2, "minutes": 10}
enum.Enum	str int	"VALUE" 10
re.Pattern	str	"\w+"

About contributing

You will make py-app-properties better if you open issues or create pull requests with improvements.