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Slides
Exceptions
Ref. WGR Chapter 6, Control-flow techniques, Section 6.4, Error handling and exceptions
"exception" is a nice way of saying "error"
Some common exceptions
RuntimeError
NoMethodError
IOError
ArgumentError
See? They're all errors! :-)
The Ruby Exception Hierarchy
Exception
NoMemoryError
ScriptError
LoadError
NotImplementedError
SyntaxError
SignalException
Interrupt
StandardError
ArgumentError
IOError
EOFError
IndexError
LocalJumpError
NameError
NoMethodError
RangeError
FloatDomainError
RegexpError
RuntimeError
SecurityError
SystemCallError
SystemStackError
ThreadError
TypeError
ZeroDivisionError
SystemExit
fatal
plain rescue
begin
x = 100 / y
rescue
x = 0
end
- if
y
is 0, then it will raise aZeroDivisionError
- but the
rescue
block will save us and makex
be 0 instead
type-specific rescue
- You can also specify what type of exception you want to rescue
- Also, let's refactor, Ruby style!
x = begin
100 / y
rescue ZeroDivisionError
0
end
- Remember, every statement in Ruby returns a value, including
begin
i want a raise
def even?(x)
raise ArgumentError, "I need a number" unless x.is_a? Numeric
x % 2 == 0
end
custom exception classes
class Whiz
class NoJuice < StandardError
end
def bang
unless power >= 1.21
raise NoJuice,
"need at least 1.21 jigawatts, but only had #{power}"
end
end
end
multiple type-specific rescue
s
def divide_if_even(x, y)
if even?(x)
x/y
end
rescue ArgumentError
puts "hey! give me a number next time"
1
rescue ZeroDivisionError
0
end
rescue
ing the exception
x = begin
100 / y
rescue ZeroDivisionError => e
puts "oops! #{e.class}: #{e.message}"
0
end
general rescue
begin
do_something_dangerous
rescue => e
puts e.class, e.message
end
even more general rescue
begin
do_something_really_dangerous
rescue Exception => e
puts e.class, e.message
end
- the plain
rescue => e
only catchesStandardError
subclasses - generally you don't want to catch all exceptions
-
NoMemoryError
s really should stop your program dead unless you know better
-
the full rescue
song and dance
begin
do_something_dangerous
rescue SomeError => e
fix_some_error(e)
rescue SomeOtherError => e
fix_some_other_error(e)
rescue => e
fix_unknown_standard_error(e)
else
something_dangerous_finished
ensure
whether_or_not_an_error_was_raised
end
Outline
- Exceptions
- "exception" is a nice way of saying "error"
- Some common exceptions
- The Ruby Exception Hierarchy
- plain `rescue`
- type-specific `rescue`
- i want a `raise`
- custom exception classes
- multiple type-specific `rescue`s
- `rescue`ing the exception
- general `rescue`
- even more general `rescue`
- the full `rescue` song and dance