implicit currying for output_epsilon and output_mu (#1374)

doc/docs/Python_User_Interface.md

@@ -2732,32 +2732,38 @@ suffix).
 <a id="output_epsilon"></a>
 
 ```python
-def output_epsilon(sim, *step_func_args, **kwargs):
+def output_epsilon(sim=None, *step_func_args, **kwargs):
 ```
 
 <div class="function_docstring" markdown="1">
 
-Given a frequency `frequency`, (provided as a keyword argument) output ε (relative
-permittivity); for an anisotropic ε tensor the output is the [harmonic
-mean](https://en.wikipedia.org/wiki/Harmonic_mean) of the ε eigenvalues. If
+Given a frequency `frequency`, (provided as a keyword argument) output $\varepsilon$ (relative
+permittivity); for an anisotropic $\varepsilon$ tensor the output is the [harmonic
+mean](https://en.wikipedia.org/wiki/Harmonic_mean) of the $\varepsilon$ eigenvalues. If
 `frequency` is non-zero, the output is complex; otherwise it is the real,
-frequency-independent part of ε (the $\omega\to\infty$ limit).
+frequency-independent part of $\varepsilon$ (the $\omega\to\infty$ limit).
+When called as part of a [step function](Python_User_Interface.md#controlling-when-a-step-function-executes),
+the `sim` argument specifying the `Simulation` object can be omitted, e.g.
+`sim.run(mp.at_beginning(mp.output_epsilon(frequency=1/0.7)),until=10)`.
 
 </div>
 
 <a id="output_mu"></a>
 
 ```python
-def output_mu(sim, *step_func_args, **kwargs):
+def output_mu(sim=None, *step_func_args, **kwargs):
 ```
 
 <div class="function_docstring" markdown="1">
 
-Given a frequency `frequency`, (provided as a keyword argument) output μ (relative
-permeability); for an anisotropic μ tensor the output is the [harmonic
-mean](https://en.wikipedia.org/wiki/Harmonic_mean) of the μ eigenvalues. If
+Given a frequency `frequency`, (provided as a keyword argument) output $\mu$ (relative
+permeability); for an anisotropic $\mu$ tensor the output is the [harmonic
+mean](https://en.wikipedia.org/wiki/Harmonic_mean) of the $\mu$ eigenvalues. If
 `frequency` is non-zero, the output is complex; otherwise it is the real,
-frequency-independent part of μ (the $\omega\to\infty$ limit).
+frequency-independent part of $\mu$ (the $\omega\to\infty$ limit).
+When called as part of a [step function](Python_User_Interface.md#controlling-when-a-step-function-executes),
+the `sim` argument specifying the `Simulation` object can be omitted, e.g.
+`sim.run(mp.at_beginning(mp.output_mu(frequency=1/0.7)),until=10)`.
 
 </div>
 

python/simulation.py

@@ -4431,14 +4431,20 @@ def _output_png(sim, todo):
     return _output_png
 
 
-def output_epsilon(sim,*step_func_args,**kwargs):
+def output_epsilon(sim=None,*step_func_args,**kwargs):
     """
-    Given a frequency `frequency`, (provided as a keyword argument) output ε (relative
-    permittivity); for an anisotropic ε tensor the output is the [harmonic
-    mean](https://en.wikipedia.org/wiki/Harmonic_mean) of the ε eigenvalues. If
+    Given a frequency `frequency`, (provided as a keyword argument) output $\varepsilon$ (relative
+    permittivity); for an anisotropic $\varepsilon$ tensor the output is the [harmonic
+    mean](https://en.wikipedia.org/wiki/Harmonic_mean) of the $\varepsilon$ eigenvalues. If
     `frequency` is non-zero, the output is complex; otherwise it is the real,
-    frequency-independent part of ε (the $\\omega\\to\\infty$ limit).
+    frequency-independent part of $\varepsilon$ (the $\\omega\\to\\infty$ limit).
+    When called as part of a [step function](Python_User_Interface.md#controlling-when-a-step-function-executes),
+    the `sim` argument specifying the `Simulation` object can be omitted, e.g.,
+    `sim.run(mp.at_beginning(mp.output_epsilon(frequency=1/0.7)),until=10)`.
     """
+    if sim is None:
+        return lambda sim: mp.output_epsilon(sim, *step_func_args, **kwargs)
+
     frequency = kwargs.pop('frequency', 0.0)
     omega = kwargs.pop('omega', 0.0)
     if omega != 0:
@@ -4447,14 +4453,20 @@ def output_epsilon(sim,*step_func_args,**kwargs):
     sim.output_component(mp.Dielectric,frequency=frequency)
 
 
-def output_mu(sim,*step_func_args,**kwargs):
+def output_mu(sim=None,*step_func_args,**kwargs):
     """
-    Given a frequency `frequency`, (provided as a keyword argument) output μ (relative
-    permeability); for an anisotropic μ tensor the output is the [harmonic
-    mean](https://en.wikipedia.org/wiki/Harmonic_mean) of the μ eigenvalues. If
+    Given a frequency `frequency`, (provided as a keyword argument) output $\mu$ (relative
+    permeability); for an anisotropic $\mu$ tensor the output is the [harmonic
+    mean](https://en.wikipedia.org/wiki/Harmonic_mean) of the $\mu$ eigenvalues. If
     `frequency` is non-zero, the output is complex; otherwise it is the real,
-    frequency-independent part of μ (the $\\omega\\to\\infty$ limit).
+    frequency-independent part of $\mu$ (the $\\omega\\to\\infty$ limit).
+    When called as part of a [step function](Python_User_Interface.md#controlling-when-a-step-function-executes),
+    the `sim` argument specifying the `Simulation` object can be omitted, e.g.,
+    `sim.run(mp.at_beginning(mp.output_mu(frequency=1/0.7)),until=10)`.
     """
+    if sim is None:
+        return lambda sim: mp.output_mu(sim, *step_func_args, **kwargs)
+
     frequency = kwargs.pop('frequency', 0.0)
     omega = kwargs.pop('omega', 0.0)
     if omega != 0: