Do more work to prevent iterator invalidation for Dict's?

[johnmyleswhite]

Should we try to do more to prevent iterator invalidation for Dict's (and possibly for other types as well)?

Contrast this Julia code:

  _       _ _(_)_     |  A fresh approach to technical computing
  (_)     | (_) (_)    |  Documentation: http://docs.julialang.org
   _ _   _| |_  __ _   |  Type "help()" for help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 0.4.0-dev+4878 (2015-05-17 18:08 UTC)
 _/ |\__'_|_|_|\__'_|  |  Commit b3116d4 (0 days old master)
|__/                   |  x86_64-apple-darwin14.3.0

julia> x = Dict("a" => 1, "b" => 2)
Dict{ASCIIString,Int64} with 2 entries:
  "b" => 2
  "a" => 1

julia> for (k, v) in x
           println((k, v))
           delete!(x, "a")
       end
("b",2)
ERROR: UndefRefError: access to undefined reference
 in next at dict.jl:721
 in anonymous at no file

with this Python code:

Python 2.7.9 |Anaconda 1.8.0 (x86_64)| (default, Dec 15 2014, 10:37:34) 
[GCC 4.2.1 (Apple Inc. build 5577)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
Anaconda is brought to you by Continuum Analytics.
Please check out: http://continuum.io/thanks and https://binstar.org
>>> x = {"a": 1, "b": 2}
>>> 
>>> for k, v in x.iteritems():
...     print((k, v))
...     del x["b"]
... 
('a', 1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: dictionary changed size during iteration
>>> 

I would argue that Python's error is more helpful for debugging, although producing such an error requires making the iteration protocol do more run-time checking on every iteration to see whether the iterator has been invalidated.

Was this a conscious decision already made for the language? Skipping these kinds of checks does feel like the Julian way, but I wanted to confirm that this was intended behavior.

[JeffBezanson]

See also #9573 and #9595

[StefanKarpinski]

Also #10116, which improved this situation.

[StefanKarpinski]

Ordered dicts might help this since modifying a dict does not change the iteration order in arbitrary ways. Especially if reassigning an existing key leaves it in the same order in the dict. It would take some careful design, but it might be possible to make it safe to mutate a dict while iterating it.