Raise an exceptionAs a Python developer you can choose to throw an exception if a condition occurs. Show
To throw (or raise) an exception, use the ExampleRaise an error and stop the program if x is lower than 0: x = -1 if x < 0: Try it Yourself » The You can define what kind of error to raise, and the text to print to the user. ExampleRaise a TypeError if x is not an integer: x = "hello" if not type(x) is int: Try it Yourself » When writing a compiler, it's hard to get exceptions right. Exceptions seem straightforward: just unwind the stack until you hit a handler--right? No. That's "not even wrong." Exceptions are not fundamentally built upon stacks. (After all, stackless implementations still have exception-handling.) Trying to understand exceptions in terms of the stack is like trying to explain the world in purely Newtonian terms: it's a reasonable model, but it fails to capture (rare but important) corner-case behavior. A "quantum" understanding of exceptions requires escape continuations. Exceptions interact nontrivially with other language features like When you throw in Consider some Python: def f(): try: return 10 finally: print("got here!") return 20 The function above returns 10, but prints "got here!" def f(): try: raise Exception() except: return 10 finally: print("got here!") return 20 The function above returns 10, but still prints "got here!" def f(): try: raise Exception() except: return 10 finally: print("got here!") return 20 The function above returns 20 (and still prints "got here!") while True: try: continue finally: print "got here!" This is an infinite loop that prints "got here!" forever. In this article, I'll explain how to implement To understand exceptions is to implement exceptions. The provided code uses
Racket macros to add all of these features to the language. It desugars them in terms of a single construct: (The techniques are not Racket-specific; for example, if you're compiling to C, you can fake My basic model for exception-handling is lifted directly from Andrew Appel's masterwork, Compiling with Continuations. Why Python and Racket?My undergrad compilers class has to implement a compiler for Python. The intermediate representation used by their compiler is a cut-down Racket extended with Pythonic control-flow constructs. That's why you'll find motivating examples in Python that get desugared into a Racket-like code. A note on call/ec This code makes heavy use of Not all languages support escape continuations, but many support the (more general) full continuations. Full continuations work in place of escape continuations with a mild performance penalty. You can also simulate Implementing return Newcomers to functional programming are often struck by the lack of a There are natural coding patterns that exploit the ability to bail out of a function early, like the following: def f(): if easy case: return this if also easy case: return that do something complicated down here Fortunately, it's easy to add a return statement to functional languages that support continuations. A return statement is a "second-class escape continuation." Conversely, think of an escape continuation as a first-class return statement. That is, imagine that In Racket, we can grab the escape continuation for a function with If we want to add a (define/return (max a b) (when (> a b) (return a)) (return b)) we can create a macro that desugars it into: (define (max a b) (call/ec (lambda (return) (when (> a b) (return a)) (return b)))) In Racket: (define-syntax (define/return stx) (syntax-case stx () [(_ f-params body ...) ; => (with-syntax ([return (datum->syntax #'f-params 'return)]) #'(define f-params (call/ec (λ (return) body ...))))])) It's just as easy to add a Implementing while A In fact, it is. But,
Fortunately, It's also best to implement these features as escape continuations. Take the general while cond: body else: otherwise This would be desugared in Racket as: (call/ec (λ (break) (letrec ([loop (λ () (when cond (call/ec (λ (continue) body)) (loop)))]) (loop) otherwise))) A call to A call to A macro to transform (define-syntax (while stx) (syntax-case stx () [(_ cond body else) ; => (with-syntax ([break (datum->syntax #'body 'break)] [continue (datum->syntax #'body 'continue)]) #'(call/ec (λ (break) (letrec ([loop (λ () (when cond (call/ec (λ (continue) body)) (loop)))]) (loop) else))))])) Implementing tryExceptions are the canonical example of escape continuations. A reasonable approach for implementing exceptions is to create a special "handler" cell in memory. That cell contains the escape continuation to invoke once an exception is raised. Let's start by compiling a We're going to add two global procedures to help-- (define $current-handler (lambda (ex) (error "top-level exception!"))) (define (current-handler) $current-handler) (define (set-current-handler! handler) (set! $current-handler handler)) The variable An isolated desugaring of the (let ([$old (current-handler)]) (call/ec (lambda (ec) (set-current-handler! (lambda (ex) (set-current-handler! $old) (ec (handler ex)))) (let ([rv body]) (set-current-handler! $old) rv)))) This code:
Whether the body returns naturally, or by invoking an exception, the old handler gets restored. But, what happens if we have code like the following? def f(): try: return 10 except: print("you should never see me") This function leaves the wrong handler installed after it returns! This is because To fix this, we need to bind new versions of (let ([$old (current-handler)]) (let* ([return (λ args (set-current-handler! $old) (apply return args))] [continue (λ () (set-current-handler! $old) (continue))] [break (λ () (set-current-handler! $old) (break))]) (call/ec (λ (ec) (set-current-handler! (λ (ex) (set-current-handler! $old) (ec (handler ex)))) (let ([rv body]) (set-current-handler! $old) rv))))) But, if we want to allow a The It must run whether we return out of it, whether we break out of it, whether we continue out of it, or where we raise out of it. But, what it must do after the
We can make all of this possible by creating a pointer to a thunk that should perform the post-finally action. We need to capture and redirect all escaping continuations to pass through There's a further wrinkle in that if during the course of the exception handling, we throw another exception, we still have to run through the In other words, the exception handler has to install an exception handler. Here's the desugaring to make it all happen: (let* ([$val (void)] [$fin (λ () $val)] [$old (current-handler)]) (call/ec (λ (fin) (let* ([return (λ args (set! $fin (λ () (apply return args))) (fin))] [continue (λ () (set! $fin continue) (fin))] [break (λ () (set! $fin break) (fin))]) (call/ec (λ (ec) (set-current-handler! (λ (ex) ; if the handler ; throws an exception, ; re-throw it after ; the finally block: (set-current-handler! (λ (ex*) (set! $fin (λ () (throw ex*))) (ec #f))) (ec (let ([rv (handler ex)]) (set! $fin (λ () rv)))))) (let ([rv body]) (set! $fin (λ () rv)) (fin))))))) (set-current-handler! $old) (set! $val finally) ($fin))) We also need to consider a (let* ([$val (void)] [$fin (λ () $val)] [$old (current-handler)]) (call/ec (λ (fin) (let* ([return (λ args (set! $fin (λ () (apply return args))) (fin))] [continue (λ () (set! $fin continue) (fin))] [break (λ () (set! $fin break) (fin))]) (call/ec (λ (ec) (set-current-handler! (λ (ex) ; re-throw after finally: (set! $fin (λ () (throw ex))) (fin))) (let ([rv body]) (set! $fin (λ () rv)) (fin))))))) (set-current-handler! $old) (set! $val finally) ($fin))) Implementing throw Compared to all the excitement above, (define (throw ex) ((current-handler) ex)) Code Macro-expanders for all of the above are available in More resources
Can we return value in exception in Python?Note that when you return an Exception class object, you'll get a representation of its associated value, usually the first item in its list of arguments. In the example above, this is the string explanation of the exception. In some cases, it may be a tuple with other information about the exception.
Can you return after a raise Python?You can't raise and return , but you could return multiple values, where the first is the same as what you're currently using, and the second indicates if an exception arose return True, sys. exc_info() == (None, None, None) or something similar but better suited to context.
How do you raise a return exception in Python?As a Python developer you can choose to throw an exception if a condition occurs. To throw (or raise) an exception, use the raise keyword.
How do you raise an exception and exit in Python?ways to do so in Python.. raise SystemExit() We can exit from Python code by raising a SystemExit exception: print("Done.") ... . sys. exit() ... . exit() and quit() exit() and quit() are “extra builtins” added by Python's site module. ... . Ctrl-D (from the REPL) Ctrl-D is the universal keyboard shortcut for exit. ... . os. _exit(). |