7.4 The try statement
Description: The try statement
Keywords: ref

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7.4 The `try` statement

The `try` statement specifies exception handlers and/or cleanup code for a group of statements:

  `try_stmt` | ::= | `try_exc_stmt | try_fin_stmt`
  `try_exc_stmt` | ::= | `"try" ":" suite`
  |  | `("except" [expression ["," target]] ":" suite)+`
  |  | `["else" ":" suite]`
  `try_fin_stmt` | ::= | `"try" ":" suite "finally" ":" suite`

  Download entire grammar as text.

There are two forms of `try` statement: `try`...`except` and `try`...`finally`. These forms cannot be mixed (but they can be nested in each other).

The `try`...`except` form specifies one or more exception handlers (the `except` clauses). When no exception occurs in the `try` clause, no exception handler is executed. When an exception occurs in the `try` suite, a search for an exception handler is started. This search inspects the except clauses in turn until one is found that matches the exception. An expression-less except clause, if present, must be last; it matches any exception. For an except clause with an expression, that expression is evaluated, and the clause matches the exception if the resulting object is ``compatible'' with the exception. An object is compatible with an exception if it is either the object that identifies the exception, or (for exceptions that are classes) it is a base class of the exception, or it is a tuple containing an item that is compatible with the exception. Note that the object identities must match, i.e. it must be the same object, not just an object with the same value.

If no except clause matches the exception, the search for an exception handler continues in the surrounding code and on the invocation stack.

If the evaluation of an expression in the header of an except clause raises an exception, the original search for a handler is canceled and a search starts for the new exception in the surrounding code and on the call stack (it is treated as if the entire `try` statement raised the exception).

When a matching except clause is found, the exception's parameter is assigned to the target specified in that except clause, if present, and the except clause's suite is executed. All except clauses must have an executable block. When the end of this block is reached, execution continues normally after the entire try statement. (This means that if two nested handlers exist for the same exception, and the exception occurs in the try clause of the inner handler, the outer handler will not handle the exception.)

Before an except clause's suite is executed, details about the exception are assigned to three variables in the `sys`module: `sys.exc_type` receives the object identifying the exception; `sys.exc_value` receives the exception's parameter; `sys.exc_traceback` receives a traceback object (see section 3.2) identifying the point in the program where the exception occurred. These details are also available through the `sys.exc_info()` function, which returns a tuple `(exc_type, exc_value, exc_traceback)`. Use of the corresponding variables is deprecated in favor of this function, since their use is unsafe in a threaded program. As of Python 1.5, the variables are restored to their previous values (before the call) when returning from a function that handled an exception.

The optional `else` clause is executed if and when control flows off the end of the `try` clause.7.1 Exceptions in the `else` clause are not handled by the preceding `except` clauses.

The `try`...`finally` form specifies a `cleanup' handler. The `try` clause is executed. When no exception occurs, the `finally` clause is executed. When an exception occurs in the `try` clause, the exception is temporarily saved, the `finally` clause is executed, and then the saved exception is re-raised. If the `finally` clause raises another exception or executes a `return` or `break` statement, the saved exception is lost. A `continue` statement is illegal in the `finally` clause. (The reason is a problem with the current implementation - this restriction may be lifted in the future). The exception information is not available to the program during execution of the `finally` clause.

When a `return`, `break` or `continue` statement is executed in the `try` suite of a `try`...`finally` statement, the `finally` clause is also executed `on the way out.' A `continue` statement is illegal in the `finally` clause. (The reason is a problem with the current implementation -- this restriction may be lifted in the future).

Additional information on exceptions can be found in section 4.2, and information on using the `raise` statement to generate exceptions may be found in section 6.9.

#### Footnotes

... clause.7.1
  Currently, control ``flows off the end'' except in the case of an exception or the execution of a `return`, `continue`, or `break` statement.