quality of life fix for pip users

README.md

@@ -63,6 +63,20 @@ See [Getting CMake](https://ccl.readthedocs.io/en/latest/source/installation.htm
 and [Installing SWIG](https://pypi.org/project/swig/) for instructions.
 Note that the code only supports Linux or Mac OS, but no Windows.
 
+### Optional dependencies
+
+For extended functionality, install one of the optional extra groups:
+
+```bash
+pip install pyccl[boltzmann]   # Boltzmann codes: CAMB, CLASS (classy), ISiTGR
+pip install pyccl[pt]          # Perturbation theory: FAST-PT, velocileptors
+pip install pyccl[emulators]   # Emulators: BaccoEmu, MiraTitan, Dark Emulator
+pip install pyccl[full]        # All of the above
+```
+
+Without `pyccl[boltzmann]`, the default `transfer_function` is `'eisenstein_hu'`.
+With `pyccl[boltzmann]` (or a full `conda` install), it defaults to `'boltzmann_camb'`.
+
 Once you have the code installed, you can take it for a spin!
 
 ```python

pyccl/boltzmann.py

@@ -25,8 +25,16 @@ def get_camb_pk_lin(cosmo, *, nonlin=False):
             spectrum. If ``nonlin=True``, returns a tuple \
             ``(pk_lin, pk_nonlin)``.
     """
-    import camb
-    import camb.model
+    try:
+        import camb
+        import camb.model
+    except ImportError as e:
+        raise CCLError(
+            "CAMB is required to use the 'boltzmann_camb' transfer function "
+            "but could not be imported. Install it with:\n"
+            "    pip install pyccl[boltzmann]\n"
+            "or: pip install camb"
+        ) from e
 
     # Get extra CAMB parameters that were specified
     extra_camb_params = {}
@@ -222,8 +230,15 @@ def get_isitgr_pk_lin(cosmo):
         :class:`~pyccl.pk2d.Pk2D`: Power spectrum \
             object. The linear power spectrum.
     """
-    import isitgr  # noqa: F811
-    import isitgr.model
+    try:
+        import isitgr  # noqa: F811
+        import isitgr.model
+    except ImportError as e:
+        raise CCLError(
+            "ISiTGR is required to use the 'boltzmann_isitgr' transfer "
+            "function but could not be imported. Install it with: "
+            "pip install pyccl[boltzmann] or pip install isitgr"
+        ) from e
 
     # Get extra CAMB parameters that were specified
     extra_camb_params = {}
@@ -396,7 +411,15 @@ def get_class_pk_lin(cosmo):
         :class:`~pyccl.pk2d.Pk2D`: Power spectrum object.\
             The linear power spectrum.
     """
-    import classy
+    try:
+        import classy
+    except ImportError as e:
+        raise CCLError(
+            "CLASS (classy) is required to use the 'boltzmann_class' transfer "
+            "function but could not be imported. Install it with:\n"
+            "    pip install pyccl[boltzmann]\n"
+            "or: pip install classy"
+        ) from e
 
     params = {
         "output": "mPk",

pyccl/cosmology.py

@@ -77,6 +77,16 @@ class MatterPowerSpectra(Enum):
     'emulator': lib.emulator_nlpk
 }
 
+def _camb_available():
+    try:
+        import camb  # noqa: F401
+        return True
+    except ImportError:
+        return False
+
+
+_DEFAULT_TRANSFER_FUNCTION = 'boltzmann_camb' if _camb_available() else 'eisenstein_hu'
+
 _TOP_LEVEL_MODULES = ("",)
 
 
@@ -238,7 +248,7 @@ def __init__(
             Neff=None, m_nu=0., mass_split='normal', w0=-1., wa=0.,
             T_CMB=DefaultParams.T_CMB,
             T_ncdm=DefaultParams.T_ncdm,
-            transfer_function='boltzmann_camb',
+            transfer_function=_DEFAULT_TRANSFER_FUNCTION,
             matter_power_spectrum='halofit',
             baryonic_effects=None,
             mg_parametrization=None,

pyccl/tests/test_power_mu_sigma_sigma8norm.py

@@ -39,9 +39,9 @@ def test_power_mu_sigma_sigma8norm(tf):
         assert np.allclose(pk_rat, gfac)
 
     with mock.patch.dict(sys.modules, {'isitgr': None}):
-        with pytest.raises(ModuleNotFoundError):
+        with pytest.raises(ccl.CCLError):
             get_isitgr_pk_lin(cosmo)
-    # Importing ccl without isitgr is fine.  No ModuleNotFoundError triggered.
+    # Importing ccl without isitgr is fine.  No error triggered.
     with mock.patch.dict(sys.modules, {'isitgr': None}):
         reload(ccl.boltzmann)
 

pyproject.toml

@@ -42,6 +42,11 @@ dev = [
     "pytest-cov",
 ]
 
+boltzmann = ["camb", "classy", "isitgr"]
+pt = ["velocileptors", "fast-pt"]
+emulators = ["baccoemu", "MiraTitanHMFemulator", "dark_emulator"]
+full = ["camb", "classy", "isitgr", "velocileptors", "fast-pt", "baccoemu", "MiraTitanHMFemulator", "dark_emulator"]
+
 [tool.setuptools.packages.find]
 include = ["pyccl*"]
 

readthedocs/source/installation.rst

@@ -43,6 +43,23 @@ Once you have ``CMake``, simply run:
    $ pip install pyccl
 
 
+Installing with optional extras
+--------------------------------
+
+CCL supports four optional dependency groups installable as pip extras:
+
+.. code-block:: bash
+
+   $ pip install pyccl[boltzmann]   # CAMB, CLASS (classy), ISiTGR
+   $ pip install pyccl[pt]          # FAST-PT and velocileptors
+   $ pip install pyccl[emulators]   # BaccoEmu, MiraTitan HMF, Dark Emulator
+   $ pip install pyccl[full]        # All of the above
+
+Without ``pyccl[boltzmann]``, the default ``transfer_function`` is
+``'eisenstein_hu'``. With it (or a ``conda`` install), the default is
+``'boltzmann_camb'``.
+
+
 Google Colab
 ============
 
@@ -61,6 +78,8 @@ To install ``pyccl`` on https://colab.research.google.com then one way is the fo
 Getting a Boltzmann Code
 ========================
 
+These packages are included in ``pip install pyccl[boltzmann]``.
+
 In order to use CCL with a Boltzmann code, you will need the ``Python`` wrappers
 for either ``CLASS`` or ``CAMB``.
 
@@ -112,6 +131,8 @@ should already be in your environment.
 Getting PT packages
 ===================
 
+These packages are included in ``pip install pyccl[pt]``.
+
 Getting FAST-PT
 ---------------
 
@@ -142,6 +163,8 @@ See full instructions in the ``velocileptors``
 Getting emulators
 =================
 
+These packages are included in ``pip install pyccl[emulators]``.
+
 The following emulators with external dependencies are currently supported
 in CCL.