A colleague ran into the following error after installing a fresh copy of Matplotlib on OSX 10.11:

$ python -c 'import matplotlib.pyplot'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/pyplot.py", line 29, in <module>
    import matplotlib.colorbar
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/colorbar.py", line 34, in <module>
    import matplotlib.collections as collections
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/collections.py", line 27, in <module>
    import matplotlib.backend_bases as backend_bases
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/backend_bases.py", line 62, in <module>
    import matplotlib.textpath as textpath
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/textpath.py", line 15, in <module>
    import matplotlib.font_manager as font_manager
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/font_manager.py", line 1420, in <module>
    _rebuild()
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/font_manager.py", line 1405, in _rebuild
    fontManager = FontManager()
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/font_manager.py", line 1043, in __init__
    self.ttffiles = findSystemFonts(paths) + findSystemFonts()
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/font_manager.py", line 323, in findSystemFonts
    for f in get_fontconfig_fonts(fontext):
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/font_manager.py", line 275, in get_fontconfig_fonts
    stderr=subprocess.PIPE)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 710, in __init__
    errread, errwrite)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 1334, in _execute_child
    child_exception = pickle.loads(data)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pickle.py", line 1382, in loads
    return Unpickler(file).load()
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pickle.py", line 858, in load
    dispatch[key](self)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pickle.py", line 966, in load_string
    raise ValueError, "insecure string pickle"
ValueError: insecure string pickle

Not surprisingly, the problem turns out to be in get_fontconfig_fonts where the command subprocess.Popen(['fc-list', '--format=%{file}\\n'], ...) returns a ValueError instead of the expected OSError, IOError and so the routine tries to load a pickle from the error message text and fails. fc-list does not exist on my colleague's system.

Probably the same as #5314.

I think your assessment isn't quite right though. subprocess.Popen(['fc-list', '--format=%{file}\\n'], ...) isn't raising a ValueError that is causing matplotlib to try to load a pickle. subprocess itself is trying to load a pickle containing an exception from a child process, and that pickle is broken, probably due to the pipe not getting properly flushed.

There's something fishy here, but I don't think matplotlib catching a ValueError is the right thing to do. According to the docs, that should only be raised when invalid arguments are passed to Popen, which they aren't here.

I found other reports of similar bugs: this seems to be the new symptom for "file not found" on OSX 10.11, which is definitely broken behavior:

rg3/youtube-dl#6840
healpy/healpy#284

Also, for fun:

https://i.imgur.com/To3DQ6J.jpg

Does:

import subprocess
p = subprocess.Popen(['foo'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
p.communicate()

also give the same error? If so, definitely a bug that should be reported to CPython and/or Apple.

That is an awesome cartoon!

I found it on the Python bugtracker. https://bugs.python.org/issue18172

Further feedback from my 10.11 source:

$ /usr/bin/python
Python 2.7.10 (default, Aug 22 2015, 20:33:39) 
[GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.0.59.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import subprocess
>>> p = subprocess.Popen(['foo'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 710, in __init__
    errread, errwrite)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 1334, in _execute_child
    child_exception = pickle.loads(data)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pickle.py", line 1382, in loads
    return Unpickler(file).load()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pickle.py", line 858, in load
    dispatch[key](self)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pickle.py", line 966, in load_string
    raise ValueError, "insecure string pickle"
ValueError: insecure string pickle

$ python3.5
Python 3.5.0 (v3.5.0:374f501f4567, Sep 12 2015, 11:00:19) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import subprocess
>>> p = subprocess.Popen(['foo'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/subprocess.py", line 950, in __init__
    restore_signals, start_new_session)
  File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/subprocess.py", line 1540, in _execute_child
    raise child_exception_type(errno_num, err_msg)
FileNotFoundError: [Errno 2] No such file or directory: 'foo'

Great. I'd say it's definitely a bug in Python 2.7.10 and/or an incompatiblity between how Apple builds Python 2.7.10 and El Capitain. I'd report this to CPython and/or Apple. I'd file the report myself, but since I don't have El Capitain to reproduce on, I'm afraid I won't be of much use.

I can't reproduce it in any way using neither system python nor python 2.7 or 3.5 from homebrew on 10.11.1. Could it be fixed in the 10.11.1 update?

@jenshnielsen - is it possible that you have fc-list installed on your system? In that case I think you would not get the error.

Yes I have font-config installed. What I meant was that I can't reproduce the issue with a non existing command such as 'foo' with either python version i.e. running

p = subprocess.Popen(['foo'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)

raises a FileNotFoundError as expected.

Ah - sorry for my confusion. My colleague also has 10.11.1. Can you think of any other explanation for the difference.

Just a shot in the dark but could it be something like a non ascii username/path

I don't think that's the case for my colleague's machine. We just upgraded a machine, and don't get the failure either.

Nicholas Angelides is my colleague with this bug - we sat down with his computer for a few minutes.

This expanded the confusion. We replicated the behavior above, both on his account and a new Test account, with system Python. Then we rebooted the machine and got the expected behavior on his account and the new Test account. I'm a bit lost for ways to proceed. Any suggestions?

Hurrah, a Heisenbug!

It appears python is assuming that a single read() call will read all available data (until EOF). That is not so. This should fix it:

% diff -u /System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py lib/python2.7/subprocess.py
--- /System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py    2015-08-22 20:36:56.000000000 -0700
+++ lib/python2.7/subprocess.py 2015-12-01 14:05:48.000000000 -0800
@@ -1313,7 +1305,13 @@

                 # Wait for exec to fail or succeed; possibly raising exception
                 # Exception limited to 1M
-                data = _eintr_retry_call(os.read, errpipe_read, 1048576)
+                data = ""
+                buf = _eintr_retry_call(os.read, errpipe_read, 1048576)
+                data += buf
+                while buf:
+                  buf = _eintr_retry_call(os.read, errpipe_read, 1048576)
+                  data += buf
+
             finally:
                 if p2cread is not None and p2cwrite is not None:
                     _close_in_parent(p2cread)

That needs to be reported upstream, we are not going to ship a monkey patch to a core library.

Yep, wasn't suggesting that... just giving people a workaround until it is resolved.

Seems nobody has reported it to Python. Here it is: http://bugs.python.org/issue26083.

A good workaround for matplotlib would be to use subprocess32 instead of subprocess when running on Python 2. It is better in too many ways to list here (and acts as a drop in replacement). The upstream issue has been fixed in 2.7.

But I wouldn't expect Apple to fix anything in a timely manner. Pro tip: Build and install your own Python, never use the one shipped by your OS distro.

I don't see the 2.7.12 release out yet . What is the best way to work around this issue to run matplotlib on El Capitan?

I'm also having this issue, and my Python is installed through HomeBrew.

Why not go with @gpshead advice instead of waiting for a fix from Python?

➜  ~  python
Python 2.7.11 (default, Jan 22 2016, 08:29:18) 
[GCC 4.2.1 Compatible Apple LLVM 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import subprocess
>>> p = subprocess.Popen(['foo'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 710, in __init__
    errread, errwrite)
  File "/usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 1334, in _execute_child
    child_exception = pickle.loads(data)
  File "/usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pickle.py", line 1388, in loads
    return Unpickler(file).load()
  File "/usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pickle.py", line 864, in load
    dispatch[key](self)
  File "/usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7/lib/python2.7/pickle.py", line 972, in load_string
    raise ValueError, "insecure string pickle"
ValueError: insecure string pickle

@acer9997 pull requests are always welcome.

2.0 will require subprocess32 to work around this bug (see #6576.)