Managing apsimNGpy project environment

If you have multiple Python projects using apsimNGpy, or you want to pin a project to a specific APSIM NG build, use a .env file for each project settings.

Prerequisite

pip install python-dotenv
pip install uv
mkdir my_project
cd my_project
uv venv # activate as required
uv init
uv pip install apsimNGpy

Step 1 — Create .env in your project root

Add your local APSIM bin path and the build or release No you expect:

The bin path is the one extracted after APSIM install, if you are going to port the project to another person indicate it and provide .env.example file, as this path is different for every computer

The content of the file might look as follows but customize it the way you want it to be.

.env

 # Path to the installed APSIM NG's bin folder (quote if it has spaces); use forward slashes on macOS/Linux.
 APSIM_BIN="C:\Program Files\APSIM2025.8.7837.0\bin" #replace with your path

 # Expected APSIM build (for checks)
 APSIM_VERSION=2025.8.7837.0

Step 2 — Load and enforce in code

config.py

 from apsimNGpy.core.config import set_apsim_bin_path, get_apsim_bin_path, apsim_version
 from dotenv import load_dotenv
 from pathlib import Path
 import os

 # load env
 load_dotenv()
 # get current bin path
 CUR_BIN_PATH  = get_apsim_bin_path()


 # get bin bath
 env_BIN_PATH  = os.getenv('APSIM_BIN', None)

 # decide whether to exit and raise if env bin path is not valid

 class ConfigurationError(Exception): # u can also use apsimNgpy built in exceptions
     pass
 if not os.path.exists(env_BIN_PATH):
   raise ConfigurationError('APSIM bin path provided in the .env file is not valid. please try again')
   # alternative is to exit
   import sys
   sys.exit(0)

 # logic for setting bin if valid
 if Path(env_BIN_PATH) != Path(CUR_BIN_PATH)
    set_apsim_bin_path(env_BIN_PATH)

Set 3.Use it in your app:

Make sure you import it in your app, such that the rules are enforced and everytime you run that project, it uses the specified path

from config import APSIM_BIN
print("APSIM bin:", APSIM_BIN)

Highlight

apsimNGpy version 1.5.0 introduces a lazy loader (Apsim) for apsimNGpy modules dependent on .NET environment and a valid APSIM bin path.

By using Apsim, we can manage our APSIM path as follows:

Use the bin path in the context manager as follows.

from apsimNGpy import Apsim
with Apsim("C:/APSIM/2025.05.1234/bin") as apsim:
    # All CLR and APSIM assemblies are resolved from this bin path
    with apsim.ApsimModel('Wheat') as model:
      model.run()

Use the env file in the context manager as follows

from apsimNGpy import Apsim
with Apsim(dotenv_path = './config/.env', bin_key ='APSIM_BIN') as apsim: # assumes that .env is in the config directory
    with apsim.ApsimModel('Wheat') as model:
    model.run()

Highlight

All modules that require .Net or apsim binary path are loaded after initializing Apsim class

Apsim can also be used without with key word as follows:

from apsimNGpy import Apsim
apsim = Apsim(dotenv_path = './config/.env', bin_key ='APSIM_BIN') # assumes that .env is in the config directory
with apsim.ApsimModel('Wheat') as model:
    model.run()

Although the Apsim class allows you to pass a bin_path or load it from a .env file, this is not strictly required. If the APSIM executable path has already been defined in config.ini or stored in a globally accessible environment variable, the class will automatically resolve and use that path. In this case, you can instantiate the Apsim class directly without specifying any path arguments. See the example below.

from apsimNGpy import Apsim
apsim = Apsim()
with apsim.ApsimModel('Wheat') as model:
    model.run()

All the apsimNGpy objects exposed via Apsim class are as follows:

ApsimModel (apsimNGpy.core.apsim)
MultiCoreManager (apsimNGpy.core.mult_cores)
run_apsim_by_path() (apsimNGpy.core.runner)
run_sensitivity() (apsimNGpy.sensitivity.sensitivity)
ConfigProblem (apsimNGpy.sensitivity.sensitivity)
ExperimentManager (apsimNGpy.core.experiment)
SensitivityManager (apsimNGpy.core.sensitivitymanager)