Creating and Understanding Tracebacks
Problem
You are debugging a program, and need to understand the stack traces that come with Ruby exceptions. Or you need to see which path the Ruby interpreter took to get to a certain line of code.
Solution
You can call the Kernel#caller method at any time to look at the Ruby interpreter’s current call stack. The call stack is represented as a list of strings.
This Ruby program simulates a company with a top-down management style: one method delegates to another, which calls yet another. The method at the bottom can use caller to look upwards and see the methods that called it:
1 #!/usr/bin/ruby -w
2 # delegation.rb
3 class CEO
4 def CEO.new_vision
5 Manager.implement_vision
6 end
7 end
8
9 class Manager
10 def Manager.implement_vision
11 Engineer.do_work
12 end
13 end
14
15 class Engineer
16 def Engineer.do_work
17 puts 'How did I get here?'
18 first = true
19 caller.each do |c|
20 puts %{#{(first ? 'I' : ' which')} was called by "#{c}"}
21 first = false
22 end
23 end
24 end
25
26 CEO.new_vision
Running this program illustrates the path the interpreter takes to Engineer.do_work:
$ ./delegation.rb How did I get here? I was called by "delegation.rb:11:in 'implement_vision'" which was called by "delegation.rb:5:in 'new_vision'" which was called by "delegation.rb:26"
Discussion
Each string in a traceback shows which line of Ruby code made some method call. The first bit of the traceback given above shows that Engineer.do_work was called by Manager.implement_vision on line 11 of the program. The second line shows how Manager.implement_vision was called, and so on.
Remember the stack trace displayed when a Ruby script raises an exception? It’s the same one you can get any time by calling Kernel#caller. In fact, if you rescue an exception and assign it to a variable, you can get its traceback as an array of strings the equivalent of calling caller on the line that triggered the exception:
def raise_exception
raise Exception, 'You wanted me to raise an exception, so…'
end
begin
raise_exception
rescue Exception => e
puts "Backtrace of the exception:\n #{e.backtrace.join("\n ")}"
end
# Backtrace of the exception:
# (irb):2:in 'raise_exception'
# (irb):5:in 'irb_binding'
# /usr/lib/ruby/1.8/irb/workspace.rb:52:in 'irb_binding'
# :0
Note the slight differences between a backtrace generated from a Ruby script and one generated during an irb session.
If you’ve used languages like Python, you might long for “real” backtrace objects. About the best you can do is to parse the strings of a Ruby backtrace with a regular expression. The parse_caller method below extracts the files, lines, and method names from a Ruby backtrace. It works in both Ruby programs and irb sessions.
CALLER_RE = /(.*):([0-9]+)(:in \’(.*)’)?/
def parse_caller(l)
l.collect do |c|
captures = CALLER_RE.match(c)
[captures[1], captures[2], captures[4]]
end
end
begin
raise_exception
rescue Exception => e
puts “Exception history:”
first = true
parse_caller(e.backtrace).each do |file, line, method|
puts %{ #{first ? “L” : “because l”}ine #{line} in “#{file}”} +
%{ called “#{method}” }
first = false
end
end
# Exception history:
# Line 2 in “(irb)” called “raise_exception”
# because line 24 in “(irb)” called “irb_binding”
# because line 52 in “/usr/lib/ruby/1.8/irb/workspace.rb”
called "irb_binding" # because line 0 in "" called ""
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