Except block

As explained in Python How-To

An except clause is the handler block within a Python try...except statement that executes when a matching exception is raised in the protected try clause.

An except block handles an exception raised by the protected try block. When a matching exception occurs, Python skips the remaining try statements, runs the except block, and then continues outside the statement; when no exception occurs, Python skips it. section 12.3.1

Use except blocks to keep failure handling focused and maintainable:

• Put only code that can raise the intended exception in the try block; a narrow protected region makes the failure source easier to identify. section 12.3.1
• Name the exceptions explicitly instead of using a bare except block. section 12.3.1 section 12.6
• Use separate except blocks for unrelated exceptions; group closely related exceptions in a tuple when they share the same response. section 12.3.3 section 12.6
• Return, report, or otherwise finish the error path before code requiring successfully assigned values runs. section 12.4

Handle invalid task urgency

def process_task_string(text): title, urgency_str = text.split(",") try: urgency = int(urgency_str) except ValueError as ex: print(f"Couldn't cast the number. Description: {ex}") return None return Task(title, urgency) process_task_string("Laundry,3#") # output: Couldn't cast the number. Description: invalid literal for int() with base 10: '3#' process_task_string("Laundry,3") # returns: Task(title='Laundry', urgency=3) The successful path skips the except block. The invalid input enters it and returns None, preventing later code from using an unassigned urgency value. section 12.3.1 section 12.4

Figure 12.4 The workflow of a try...except statement. We include the code that potentially raises an exception in the try clause. When such an exception is raised, the except clause gets executed, and execution moves to the code outside the statement. If no exception is raised when the code in the try clause is executed, Python skips the except clause.

Trace the two execution paths: successful try execution skips the except block, while a raised exception transfers control to it and then moves outside the statement. section 12.3.1

How Python How-To covers it

How deep it goes

Explains control flow, narrow try scope, explicit exception types, multiple clauses, grouped exceptions, and failure-path returns.

Examples

Uses task-string parsing, integer conversion, invalid urgency input, ValueError handling, and separate or grouped exception clauses.

What it assumes

Assumes basic Python statements and functions while teaching exception control flow from the try...except pattern.

What you can do after

Apply it via Python syntax to catch named exceptions, report failures, return fallback values, and preserve successful execution.

How it connects

Connects exception handling with try statements, ValueError, NameError, else clauses, finally clauses, and logging.

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Also explained in

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  The Quick Python Book, Fourth EditionAn except clause is a branch of a Python try statement that handles exceptions matching a specified exception type.

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Related concepts

• try-catch statementpart of · A try-catch statement is a control-flow construct that executes code under exception handling and transfers control to a matching handler when an exception is raised.
• exceptionuses · An exception is a runtime signal or object representing an abnormal condition that interrupts ordinary program flow and may be handled or propagated.
• ValueErroruses · ValueError is a built-in Python exception raised when an operation receives an argument of an appropriate type but an inappropriate value.
• blockkind of · A code block is a grouped region of code treated as a unit, with boundaries and scope or execution behavior determined by its language or context.

This explainer was generated by Manning's knowledge graph from the text of Python How-To. The section links point to the parts of the book it draws on.