Repository: yt-project/unyt (python)

Issue:
"mu," fonts in matplotlib, and unicode fallbacks
The "mu" unicode character seems to have some interesting things going on in matplotlib.  I'm not issuing a pull request because I have a suspicion that there are more complex things going on than I realize.

matplotlib uses the `FT_get_glyph_index` function when it's attempting to identify a symbol, and then uses the result of that to identify the appropriate replacement for fallbacks.  Using the `cm` font system, this shows up when using unicode characters.

The subtlety seems to show up when using `\u00b5` versus `\u03bc` to represent the mu character.  In `unyt/_unit_lookup_table.py`, both b5 and3bc show up in the `unit_prefixes`.  (As a sidenote, I think evidence of the problem is visible in vim if you search for u00b5 and u03bc -- searching for the former includes the latter, but not the reverse.)  Elsewhere in the code, 00b5 shows up in things like `generate_name_alternatives` etc, but u03bc does not.

The problem comes in in that the `used_prefix` is *replaced* with u00b5.  This character returns a *positive* in `FT_get_glyph_index`, so it does *not* trigger the "replace with a symbol from STIX" code path in matplotlib, and instead thinks it has correctly identified the glyph as "grave" (backtick I think).  So it proceeds using this as the symbol instead of using the backup symbol.

What I *think* is the "pragmatic" way forward would be to swap everything for u03bc, and reverse the behavior.  Unfortunately, my reading of the codenames in Unicode (which are emphasized in `_unit_lookup_table.py` as drawn from Juan Reyero) is that the *correct* one is really u00b5, and u03bc really is what should be the more specific implementation...  So I'm not sure even what to suggest, since it kind of reads to me like we're bumping up at the ugly space between pragmatism and idealism.

Anyway here's a script that demonstrates:

```
mport matplotlib.pyplot as plt
import numpy as np
from matplotlib.style import context
style = {
    'mathtext.fontset': 'cm',
    'mathtext.fallback_to_cm': True,
}
with context(style, after_reset=False):
    labels = ['$\\rm{x\\ \\ (\\rm{' + '\u03bc' + 'm})}$', '$\\rm{y\\ \\ (\\rm{' + '\u00b5' + '})}$']
    t = np.arange(0.0, 2.0, 0.01)
    s = 1 + np.sin(2 * np.pi * t)
    fig, ax = plt.subplots()
    ax.plot(t, s)
    ax.set(xlabel=labels[0], ylabel=labels[1],
           title='About as simple as it gets, folks')
    ax.grid()
    fig.savefig("test.png")
```

Pull request description:
Switch from u00b5 to u03bc for micro
This closes #145 .

This script:

```python
import yt
ds = yt.testing.fake_amr_ds(["density"])
p = yt.SlicePlot(ds, "x", "density")
p.set_axes_unit("um")
```

demonstrates the old and new behavior; first is old, second is new.
![old_micrometer_axes](https://user-images.githubusercontent.com/89019/84918962-ed34fb00-b086-11ea-8c0d-f3b7817605bc.png)
![new_micrometer_axes](https://user-images.githubusercontent.com/89019/84918969-f0c88200-b086-11ea-856f-9190dacb3442.png)



Patch:
diff --git a/HISTORY.rst b/HISTORY.rst
index 1010b0d..13731aa 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -269,7 +269,7 @@ you run into issues please let us know by `opening an issue on GitHub
       >>> units.kilometer
       km
       >>> units.microsecond
-      µs
+      μs
 
   See `PR #68 <https://github.com/yt-project/unyt/pull/68>`_.
 * The ``unyt`` codebase is now automatically formatted by `black
diff --git a/tox.ini b/tox.ini
index 34aca05..b98e44d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -28,6 +28,7 @@ deps =
     black ; python_version >= '3.6.0'
     setuptools
     matplotlib
+    docutils
 commands =
     pytest --cov=unyt --cov-append --doctest-modules --doctest-plus --doctest-rst --basetemp={envtmpdir} -W once
     coverage report --omit='.tox/*'
diff --git a/unyt/_unit_lookup_table.py b/unyt/_unit_lookup_table.py
index 8970d6a..b71c9bb 100644
--- a/unyt/_unit_lookup_table.py
+++ b/unyt/_unit_lookup_table.py
@@ -506,8 +506,10 @@ def generate_name_alternatives():
         # Are we SI prefixable or not?
         if entry[4]:
             for prefix in unit_prefixes:
-                if prefix in ["u", "μ"]:
-                    used_prefix = "µ"
+                # This is specifically to work around
+                # https://github.com/yt-project/unyt/issues/145
+                if prefix in ["u", "μ", "µ"]:
+                    used_prefix = "μ"
                 else:
                     used_prefix = prefix
                 append_name(names[prefix + key], used_prefix + key, prefix + key)


Test patch:
diff --git a/unyt/tests/test_units.py b/unyt/tests/test_units.py
index cc37ee7..e9d5ee9 100644
--- a/unyt/tests/test_units.py
+++ b/unyt/tests/test_units.py
@@ -769,8 +769,8 @@ def test_attosecond():
 def test_micro():
     from unyt import Unit
 
-    assert str(Unit("um")) == "µm"
-    assert str(Unit("us")) == "µs"
+    assert str(Unit("um")) == "μm"
+    assert str(Unit("us")) == "μs"
 
 
 def test_show_all_units_doc_table_ops():