René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Error with linestyles rcParams entries under the form (on, off, ...) and a style context manager","articleBody":"**Bug summary**\r\n\r\nWhen defining an rcParams entry that is related to linestyle with an (on, off, ....) pattern, and using a style context manager somewhere in the code to temporarily apply other values of rcParams, an exception occurs.\r\n\r\n**Code for reproduction**\r\n\r\nHere is an example with the grid linestyle. (Related [discussion](https://gitter.im/matplotlib/matplotlib?at=5a0bbdb9ba39a53f1a9f438d) on gitter.)\r\n\r\n```python\r\nimport matplotlib.pyplot as plt\r\nfrom matplotlib import rcParams\r\n\r\nrcParams[\"grid.linestyle\"] = (4, 4)  # no error with something liked \"dotted\"\r\n\r\nfig = plt.figure()\r\n\r\nwith plt.style.context([\"default\"]):\r\n    ax0 = fig.add_subplot(121)\r\n    ax0.grid()\r\n\r\nax1 = fig.add_subplot(122)\r\nax1.grid()\r\n\r\n#fig.savefig(\"test.png\")\r\n```\r\n\r\n**Actual outcome**\r\n\r\nAn exception is raised. With Matplotlib 2.1.0 and the master branch:\r\n```\r\nValueError: Key grid.linestyle: linestyle (None, [4.0, 4.0]) is not a valid on-off ink sequence.\r\n```\r\n*See at the end of the post for the full traceback.*\r\n\r\nI think this is occurs when exiting the style context manager. It looks like the linestyle given as `(on, off, ...)` is internally processed and stored into a form `(offset, (on, off, ...))` that is actually not recognized by the line validator, which then raises an error when it is passed back to rcParams to restore the former style.\r\n\r\nPlease note that it may not even be due to the stricter linestyle validation that was introduced in Matplotlib 2.1 (#8040), as Matplotlib 2.0.2 as well as Matplotlib 1.5.3 also throw an exception, although slightly different:\r\n```\r\nValueError: You passed in an invalid linestyle, `(4, 4)`.  See docs of Line2D.set_linestyle for valid values.\r\n```\r\n\r\n**Expected outcome**\r\n\r\nNo exception when exiting the style context manager, and a plot with 2 different grid linestyles, both defined through the dedicated \"grid.linestyle\" entry in rcParams.\r\n\r\n**Matplotlib version**\r\n\r\n  * Operating system: Linux (Fedora 26)\r\n  * Matplotlib version: 2.1.0 from conda, master, 1.5.3 and 2.0.2 from the git repo\r\n  * Matplotlib backend (`print(matplotlib.get_backend())`): Qt5Agg\r\n  * Python version: 3.6.3 from conda\r\n\r\n**Miscellaneous**\r\n\r\nThe full traceback of the exception (with Matplotlib from the master branch):\r\n```\r\n---------------------------------------------------------------------------\r\nTypeError                                 Traceback (most recent call last)\r\n~/FOSS/matplotlib/lib/matplotlib/rcsetup.py in _validate_linestyle(ls)\r\n    939 \r\n--\u003e 940         return (None, validate_nseq_float()(ls))\r\n    941 \r\n\r\n~/FOSS/matplotlib/lib/matplotlib/rcsetup.py in __call__(self, s)\r\n    308                     else val\r\n--\u003e 309                     for val in s]\r\n    310         except ValueError:\r\n\r\n~/FOSS/matplotlib/lib/matplotlib/rcsetup.py in \u003clistcomp\u003e(.0)\r\n    308                     else val\r\n--\u003e 309                     for val in s]\r\n    310         except ValueError:\r\n\r\nTypeError: float() argument must be a string or a number, not 'NoneType'\r\n\r\nDuring handling of the above exception, another exception occurred:\r\n\r\nValueError                                Traceback (most recent call last)\r\n~/FOSS/matplotlib/lib/matplotlib/__init__.py in __setitem__(self, key, val)\r\n    911             try:\r\n--\u003e 912                 cval = self.validate[key](val)\r\n    913             except ValueError as ve:\r\n\r\n~/FOSS/matplotlib/lib/matplotlib/rcsetup.py in _validate_linestyle(ls)\r\n    945         raise ValueError(\"linestyle {!r} is not a valid on-off ink \"\r\n--\u003e 946                          \"sequence.\".format(ls))\r\n    947 \r\n\r\nValueError: linestyle (None, [4.0, 4.0]) is not a valid on-off ink sequence.\r\n\r\nDuring handling of the above exception, another exception occurred:\r\n\r\nValueError                                Traceback (most recent call last)\r\n\u003cipython-input-7-3b8f6f6ce3da\u003e in \u003cmodule\u003e()\r\n      8 with plt.style.context([\"default\"]):\r\n      9     ax0 = fig.add_subplot(121)\r\n---\u003e 10     ax0.grid()\r\n     11 \r\n     12 ax1 = fig.add_subplot(122)\r\n\r\n~/anaconda3/envs/matplotlib_dev/lib/python3.6/contextlib.py in __exit__(self, type, value, traceback)\r\n     86         if type is None:\r\n     87             try:\r\n---\u003e 88                 next(self.gen)\r\n     89             except StopIteration:\r\n     90                 return False\r\n\r\n~/FOSS/matplotlib/lib/matplotlib/style/core.py in context(style, after_reset)\r\n    156         yield\r\n    157     finally:\r\n--\u003e 158         mpl.rcParams.update(initial_settings)\r\n    159 \r\n    160 \r\n\r\n~/anaconda3/envs/matplotlib_dev/lib/python3.6/_collections_abc.py in update(*args, **kwds)\r\n    839             if isinstance(other, Mapping):\r\n    840                 for key in other:\r\n--\u003e 841                     self[key] = other[key]\r\n    842             elif hasattr(other, \"keys\"):\r\n    843                 for key in other.keys():\r\n\r\n~/FOSS/matplotlib/lib/matplotlib/__init__.py in __setitem__(self, key, val)\r\n    912                 cval = self.validate[key](val)\r\n    913             except ValueError as ve:\r\n--\u003e 914                 raise ValueError(\"Key %s: %s\" % (key, str(ve)))\r\n    915             dict.__setitem__(self, key, cval)\r\n    916         except KeyError:\r\n\r\nValueError: Key grid.linestyle: linestyle (None, [4.0, 4.0]) is not a valid on-off ink sequence.\r\n```","author":{"url":"https://github.com/afvincent","@type":"Person","name":"afvincent"},"datePublished":"2017-11-15T23:00:03.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/9792/matplotlib/issues/9792"}