docs: add guidance on dependency version range

doc/source/how-to/packaging.rst

@@ -400,6 +400,100 @@ Examples of PyAnsys projects that have these optional dependencies are:
 - `PyAnsys Geometry targets <https://github.com/ansys/pyansys-geometry/blob/e6d8210f9d79718d607a2f4b2e8ead33babcbfca/pyproject.toml#L44-L58>`_
 - `PyACP targets <https://github.com/ansys/pyacp/blob/f4d8c1779cd451b1fc8ef649cc3b2cd5799ff11a/pyproject.toml#L89-L110>`_
 
+Dependency version range
+------------------------
+
+.. note::
+
+    This guidance applies only to PyAnsys *library* projects. For projects
+    which deliver an *application* or *dedicated workflow*, it is
+    recommended to fully pinning all (direct and transitive) dependencies.
+
+When specifying dependencies in a project, it is generally recommended to avoid
+setting upper version limits unless it is absolutely necessary. The reason for
+that is because arbitrarily restricting a dependency to be lower than a
+certain version (for example `numpy<2.0`) can prevent your project from working
+with newer and perfectly compatible versions, and often causes more problems
+than it solves. Such restrictions limit forward compatibility, block users from
+upgrading dependencies, and increase the risk of version conflicts.
+
+This issue is even more critical in the context of the PyAnsys `metapackage`_
+which install many PyAnsys projects. In this setup, having strict upper bounds
+on versions can easily result in unsatisfiable dependency constraints across
+the ecosystem. For instance, if a package declares a dependency on `numpy<2.0`
+despite being compatible with later versions, and another package requires
+`numpy>=2.0.0` to leverage a new feature, it becomes impossible to install
+both packages simultaneously. This occurs even though no actual incompatibility
+exists between them, and it can lead to frustration for users and maintainers
+as it prevents otherwise compatible packages from being used together seamlessly.
+
+It is better to define only a minimum version (`>=`) and rely on Continuous
+Integration (CI) to detect real breakages as dependencies evolve. If a future
+version does introduce a breaking change, you can then add an upper bound with
+a clear explanation. For example:
+
+.. code-block:: toml
+
+    [project]
+    dependencies = [
+        "numpy<2.0", # breaking changes in Python and C APIs'.
+    ]
+
+Setting a lower bound (`>=`) is considered good practice for multiple reasons.
+First, it documents the oldest version of a dependency that your project
+explicitly supports. It is often the oldest version that is compatible with
+the Python versions you support. For example, if your project supports
+Python versions from `3.11` to `3.13`, you need to ensure that all dependencies
+are compatible with at least Python `3.11`. This is important for users who may
+be using older versions of Python and want to ensure compatibility with your
+project. In other cases, the lower bound is related to the version where
+certain key features your code relies on were first introduced. For instance,
+if your code uses an API or behavior that only appeared in version `1.3`,
+setting `>=1.3` communicates both a technical requirement and an implicit
+contract to your users and contributors.
+
+This helps avoiding unexpected breakages when someone installs your project
+in an environment with older versions of dependencies. Rather than encountering
+obscure runtime errors or missing features, the version constraint prevents
+your project to be installed. It also helps to maintain clarity for long-term
+maintenance and simplifies debugging.
+
+Below is an example of a dependency specification that follows these guidelines:
+
+.. tab-set::
+
+    .. tab-item:: flit
+
+        .. code-block:: toml
+
+            [project]
+            dependencies = [
+                "matplotlib>=3.5.2",
+                "numpy>=1.20.0",
+            ]
+
+    .. tab-item:: poetry
+
+        .. code-block:: toml
+
+            [tool.poetry.dependencies]
+            matplotlib = ">=3.5.2"
+            numpy = ">=1.20.0"
+
+    .. tab-item:: setuptools
+
+        .. code-block:: python
+
+            from setuptools import setup
+
+            setup(
+                ...,
+                install_requires=[
+                    "matplotlib >= 3.5.2",
+                    "numpy >= 1.20.0",
+                ],
+            )
+
 Dependabot
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