Suggestion for a new loop type

[HappyFakeboulder]

BPO	34332
Nosy	@zware, @matrixise

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-08-03.19:52:12.431>
created_at = <Date 2018-08-03.19:28:12.753>
labels = ['type-feature', '3.8']
title = 'Suggestion for a new loop type'
updated_at = <Date 2018-08-04.07:53:14.109>
user = 'https://bugs.python.org/HappyFakeboulder'

bugs.python.org fields:

activity = <Date 2018-08-04.07:53:14.109>
actor = 'matrixise'
assignee = 'none'
closed = True
closed_date = <Date 2018-08-03.19:52:12.431>
closer = 'zach.ware'
components = []
creation = <Date 2018-08-03.19:28:12.753>
creator = 'Happy Fakeboulder'
dependencies = []
files = []
hgrepos = []
issue_num = 34332
keywords = []
message_count = 3.0
messages = ['323065', '323067', '323099']
nosy_count = 3.0
nosy_names = ['zach.ware', 'matrixise', 'Happy Fakeboulder']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue34332'
versions = ['Python 3.8']

[HappyFakeboulder]

A "while-except" loop, in which it acts like a do-while loop from other languages, except the condition is "did an exception occur during the execution of the iteration". One could specify a type of exception at the top, similar to an except block header. (sorry, I'm an idiot)
Example (the keyword might be different):

# asks for a number until the user gives one properly, then
# calculates the square of it.
whexc ValueError:
num = int(input("Enter a number: "))
print("The square of your number is " + str(num * num)) # assuming whexc works, this will not cause an error

[zware]

Ideas like this are better sent to the python-ideas@python.org mailing list for discussion rather than immediately opening an issue. Note though that I think this has a low chance of acceptance; there's a very high bar to clear to add syntax, even higher for new keywords, and this doesn't seem to buy much over:

   while True:
       try:
           num = int(input("Enter a number: "))
       except ValueError:
           print("I said a *number*")
       else:
           break
   print("The square of your number is", num**2)

Especially considering that this version allows you to handle the error however you want, and handle different errors differently, rather than just silently restarting the loop no matter the error.

Also, this will be valid in 3.8 and achieve exactly what you're asking for:

   while not (ans := input("Enter a number: ")).isnumeric():
       pass
   print("The square of your number is", int(ans) ** 2)

Do feel free to send your idea to python-ideas anyway, I'm just one opinion :)

[matrixise]

You can also use a context manager and the with statement.