Clicking on Path Browser in the File menu gives an error instead of opening the Path Browser.

Error is printed in console window if IDLE started from one.
>>> import idlelib.idle
# Select pathbrowser in File menu
Exception in Tkinter callback
Traceback (most recent call last):
  File "F:\dev\3x\lib\tkinter\__init__.py", line 1699, in __call__
    return self.func(*args)
  File "F:\dev\3x\lib\idlelib\editor.py", line 671, in open_path_browser
    pathbrowser.PathBrowser(self.flist)
  File "F:\dev\3x\lib\idlelib\pathbrowser.py", line 18, in __init__
    self.init(flist)
TypeError: init() takes 1 positional argument but 2 were given

The issue is this:
1. PathBrowser subclasses ModuleBrowser.
2. ModuleBrowser.init() has the common code for initializing both.
3. #31460 changed the signature of ModuleBrowser.__init__ *and* .init.
4. I must not have tested pathbrowser after the .init change.  Bad.
5. test_pathbrowser needs a new gui test class that at least calls PathBrowser.__init__, which will call ModuleBrowser.init

3.6.3 is OK, 3.7.0a2 has this bug.  3.7.0a3 is due 11/27.  Patch should be merged before this with or without new unittests.
---

Module items can be expanded either by clicking [+] or double-clicking.  The latter also opens the file so that lines can be highlighted.  I have thought about disabling both, and opening a module browser instead, but I can see the usefulness of expanding several files one level deep to get an overview of parts or all of a package.  On the other hand, we might have double-click open both the module and a module browser.

I added a test for PathBrowser, but I didn't change the existing tests, except to move them to their own test class.

The reason double-clicking got worse from 3.6.3 to now is this.  Previously, browser.ModuleBrowserTreeItem.OnDoubleClick ignored the file_open global (set to pyshell.flist.open in .__init__) and called pyshell.flist.open.  The Class and Method OnDoubleClick called file_open.  So double-clicking a file name in pathbrowser opened the file, but expanding a file and double-clicking a class or function name raised "'NoneType' object is not callable".  I don't know if I ever tried the latter before.  We subsequently changed the Module DoubleClick to also use file_open, so double clicking a file name also raises. (I meant to submit this yesterday.)

New changeset 20d48a44a54ed5e4a6df00e89ae27e3983128265 by Terry Jan Reedy (Cheryl Sabella) in branch 'master':
bpo-32100: IDLE: Fix pathbrowser errors; improve tests. (#4484)
https://github.com/python/cpython/commit/20d48a44a54ed5e4a6df00e89ae27e3983128265

New changeset ae3c5c7b9e5e5ba53213e12cc100e32415d5762c by Terry Jan Reedy in branch '3.6':
[3.6] bpo-32100: IDLE: Fix pathbrowser errors; improve tests. (GH-4484) (#4512)
https://github.com/python/cpython/commit/ae3c5c7b9e5e5ba53213e12cc100e32415d5762c

Thanks for catching and fixing this.

New changeset fd6f8c56b9a757210e8ceaea9edc50e502dc2085 by Terry Jan Reedy in branch 'master':
bpo-32100: Delete unneeded import in idlelib.pathbrowser. (#4626)
https://github.com/python/cpython/commit/fd6f8c56b9a757210e8ceaea9edc50e502dc2085

New changeset 38ecf57306ef25874ef7c44a9875771e90faee97 by Terry Jan Reedy (Miss Islington (bot)) in branch '3.6':
bpo-32100: Delete unneeded import in idlelib.pathbrowser. (GH-4626) (#4629)
https://github.com/python/cpython/commit/38ecf57306ef25874ef7c44a9875771e90faee97