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I find it hard to argue it’s (the else block) any more useful than just using the `else` block’s logic in the try block. try: import uvloop

I find it hard to argue it’s (the else block) any more useful than just using the `else` block’s logic in the try block. try: import uvloop asyncio.set_event_loop_policy(uvloop.EventLoopPolicy) except ImportError: pass Does the handler for if the try succeeds need to be in an else, separated from the success logic?

figgis 8y ago
You can actually use else for try statements. Doesn't answer question but it's something I learned relatively recently and may help some others. try: import uvloop asyncio.set_event_loop_policy(uvloop.EventLoopPolicy) except ImportError: pass else: print("well that didn't work")
- pmontra 8y ago
  I'm not sure about what the message in that print() refers to (else fires if the import succeeded) so here's another example $ python3 >>> try: 1/0 except ZeroDivisionError: print("exception") else: print("surprise") exception >>> try: 1/1 except ZeroDivisionError: print("exception") else: print("normal") 1.0 normal The choice of the name else is unfortunate because it's activated under the same circumstances of a then branch of an "if". Maybe its name should really be "then". That would also look like a .then of a JavaScript Promise, which would make sense nowadays and was obviously impossible to foresee back in 1991. However the name "else" makes sense if it refers to "except": it's what happens when no "except" fire. I still don't understand why it should be "else" for a "for" that succeeded: it should be a "then" there.
  - saurik 8y ago
    In the case of a for loop, you are modeling "a for which did not break". The cases where you want that block are typically when you are looping through a list trying to find something, so "if I found it I broke, else do this default logic".
    - pmontra 8y ago
      That's the intended use case but it works also when there is no break $ python3 >>> for i in [1,2,3]: print(i) else: print("completed") 1 2 3 completed That should be a "then", but admittedly a very useless one. I agree that for the intended use case "else" is a better choice. It's like the else of the if that failed to find 2 in this code: >>> for i in [1,2,3]: if i == 2: break else: print("we didn't find 2") Still "else" doesn't hint much at what's going on, especially if the body of the loop is long. Maybe "nobreak" would make it immediately understandable to everybody? However in the best Python tradition I would make it very explicit, remove the feature from the language and use boolean flags. They are less compact but easier to understand than a feature that (probably) very few people know about. >>> we_found_2 = False # this is a valid assertion now >>> for i in [1,2,3]: if i == 2: we_found_2 = True break if not we_found_2: print("we didn't find 2") That's also the only way to do it if there are multiple break in the loop on different conditions.
  - figgis 8y ago
    yeah you are right, it should be print("well that's done")
abecedarius 8y ago
Generally you can't be sure that the second statement does not also raise ImportError, where you do not want to catch it.
pmontra 8y ago
By the way, try also has an else clause that behaves like the one in if https://docs.python.org/3/reference/compound_stmts.html https://docs.python.org/3/reference/compound_stmts.html
- dragonwriter 8y ago
  It behaves more like the main branch of “if” unless you construe “try” as “try to produce an exception”; outside of “if”, Python’s “else” pretty consistently implies exactly the wrong picture of normal flow.
- nerdwaller 8y ago
  I guess I wasn’t clear, because my point is generally that moving the logic to an else block in any supported control structures (if, for, try, ...) doesn’t necessarily make it clearer or cleaner.

figgis 8y ago

You can actually use else for try statements. Doesn't answer question but it's something I learned relatively recently and may help some others. try: import uvloop asyncio.set_event_loop_policy(uvloop.EventLoopPolicy) except ImportError: pass else: print("well that didn't work")

pmontra 8y ago
I'm not sure about what the message in that print() refers to (else fires if the import succeeded) so here's another example $ python3 >>> try: 1/0 except ZeroDivisionError: print("exception") else: print("surprise") exception >>> try: 1/1 except ZeroDivisionError: print("exception") else: print("normal") 1.0 normal The choice of the name else is unfortunate because it's activated under the same circumstances of a then branch of an "if". Maybe its name should really be "then". That would also look like a .then of a JavaScript Promise, which would make sense nowadays and was obviously impossible to foresee back in 1991. However the name "else" makes sense if it refers to "except": it's what happens when no "except" fire. I still don't understand why it should be "else" for a "for" that succeeded: it should be a "then" there.
- saurik 8y ago
  In the case of a for loop, you are modeling "a for which did not break". The cases where you want that block are typically when you are looping through a list trying to find something, so "if I found it I broke, else do this default logic".
  - pmontra 8y ago
    That's the intended use case but it works also when there is no break $ python3 >>> for i in [1,2,3]: print(i) else: print("completed") 1 2 3 completed That should be a "then", but admittedly a very useless one. I agree that for the intended use case "else" is a better choice. It's like the else of the if that failed to find 2 in this code: >>> for i in [1,2,3]: if i == 2: break else: print("we didn't find 2") Still "else" doesn't hint much at what's going on, especially if the body of the loop is long. Maybe "nobreak" would make it immediately understandable to everybody? However in the best Python tradition I would make it very explicit, remove the feature from the language and use boolean flags. They are less compact but easier to understand than a feature that (probably) very few people know about. >>> we_found_2 = False # this is a valid assertion now >>> for i in [1,2,3]: if i == 2: we_found_2 = True break if not we_found_2: print("we didn't find 2") That's also the only way to do it if there are multiple break in the loop on different conditions.
- figgis 8y ago
  yeah you are right, it should be print("well that's done")

abecedarius 8y ago

Generally you can't be sure that the second statement does not also raise ImportError, where you do not want to catch it.

pmontra 8y ago

By the way, try also has an else clause that behaves like the one in if https://docs.python.org/3/reference/compound_stmts.html https://docs.python.org/3/reference/compound_stmts.html

dragonwriter 8y ago
It behaves more like the main branch of “if” unless you construe “try” as “try to produce an exception”; outside of “if”, Python’s “else” pretty consistently implies exactly the wrong picture of normal flow.
nerdwaller 8y ago
I guess I wasn’t clear, because my point is generally that moving the logic to an else block in any supported control structures (if, for, try, ...) doesn’t necessarily make it clearer or cleaner.