glob.glob returns an empty list when passed an empty pattern as argument, but pathlib's Path.glob fails with IndexError.  The first option seems more reasonable (or at least it should be a ValueError).

In[6]: pathlib.Path.glob("")
Traceback (most recent call last):
  File "/home/liuchang/ENV3/lib/python3.4/site-packages/IPython/core/interactiveshell.py", line 2883, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-6-a6dcf250fe73>", line 1, in <module>
    pathlib.Path.glob("")
TypeError: glob() missing 1 required positional argument: 'pattern'

the version of my python is 3.4。
I got a TypeError， No IndexError。

>>> list(pathlib.Path().glob(""))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python3.5/pathlib.py", line 999, in glob
    selector = _make_selector(tuple(pattern_parts))
  File "/usr/lib64/python3.5/functools.py", line 458, in wrapper
    result = user_function(*args, **kwds)
  File "/usr/lib64/python3.5/pathlib.py", line 403, in _make_selector
    pat = pattern_parts[0]
IndexError: tuple index out of range
>>>

hi pitrou, should we fix it in _make_selector(pattern_parts) function? 
origin code as:

def _make_selector(pattern_parts):
    pat = pattern_parts[0]
    child_parts = pattern_parts[1:]
    if pat == '**':
        cls = _RecursiveWildcardSelector
    elif '**' in pat:
        raise ValueError("Invalid pattern: '**' can only be an entire path component")
    elif _is_wildcard_pattern(pat):
        cls = _WildcardSelector
    else:
        cls = _PreciseSelector
    return cls(pat, child_parts)

Is it a good fix that: check the length of pattern_parts, if its length < 2, we set pat to empty str, set child_parts to a empty list。

A simple code like:

def _make_selector(pattern_parts):
    try:
        pat = pattern_parts[0]
        child_parts = pattern_parts[1:]
    except IndexError:
        pat = ""
        child_parts = []
    if pat == '**':
        cls = _RecursiveWildcardSelector
    elif '**' in pat:
        raise ValueError("Invalid pattern: '**' can only be an entire path component")
    elif _is_wildcard_pattern(pat):
        cls = _WildcardSelector
    else:
        cls = _PreciseSelector
    return cls(pat, child_parts)

This should be easy to fix. It should raise ValueError.

I added a patch which causes glob to raise a ValueError exception if it is called with an empty string. I also added a test verifying the change and have run all the tests and they pass.

Though I've been reading the developer guide, I'm a bit unfamiliar with the process here so I'm not totally sure I'm doing this right. I created the patch relative to the current default branch (even though the discussion here seems to indicate it should maybe be applied going back a few versions).

New changeset 35bf4f9e68f3 by Berker Peksag in branch '3.5':
Issue #23076: Path.glob() now raises a ValueError if it's called with an
https://hg.python.org/cpython/rev/35bf4f9e68f3

New changeset 0a6290195f7c by Berker Peksag in branch 'default':
Issue #23076: Path.glob() now raises a ValueError if it's called with an
https://hg.python.org/cpython/rev/0a6290195f7c

Thanks for the patch, Thomas!

> Though I've been reading the developer guide, I'm a bit unfamiliar with the process here so I'm not totally sure I'm doing this right.

You did everything right :) I just tweaked the test a bit.

> I created the patch relative to the current default branch (even though the discussion here seems to indicate it should maybe be applied going back a few versions).

Backporting a patch is (most of the time) the core developer's responsibility. However, if the patch is going to be very different in maintenance branches (let's say 2.7 for example), it would be really helpful to attach a separate patch for 2.7.