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Magic numbers detector for Ruby source code.

Mago - magic numbers detector for Ruby

Magic numbers

Magic numbers (unnamed constants) are considered as a bad programming practice. Extracting them into constants or explaining variables usually provides the following advantages:

  • It is easier to read and understand.
  • It is easier to alter the value of the number, as it is not duplicated.
  • It may facilitate parameterization.
  • It helps to detect typos.


gem install mago


Ruby code in square.rb:

P = 3.14

r = 5
square = P * r ** 2


mago ./square.rb
./square.rb:3 detected magic number 5
./square.rb:4 detected magic number 2

Ignore specific numbers

Use --ignore or -i option to ignore specific numbers. By default 0 and 1 are ignored.

mago -i 2,3 ./square.rb
./square.rb:3 detected magic number 5

Show source code

Use --source or -s option to show line of source code where magic number was found.

mago -s ./square.rb
./square.rb:3| r = 5
./square.rb:4| square = P * r ** 2

Color output

Use --color or -c option to colorize output.

Using API

See complete documentation at rubydoc. Here is a simple example:

require 'mago'

# Initialize detector with ruby files and options
detector =['./square.rb', './math/fibonacci.rb'], :ignore => [1,2,3])

# Run detector it to build a report
report =  # => #<Mago::Report ...>

# Use report as you want. The following code provides an output like this:
#  ./square.rb
#      Line 3:  5
#      Line 6:  0
#  ./math/fibonacci.rb
#      Line 1:  0.0
#      Line 6:  5.0
report.files.each do |file|
  puts file.path
  file.magic_numbers.each do |number|
    puts "    Line #{number.line}:  #{number.value}"

report.errors.each do |error|
  puts "ERROR: #{error}"


Copyright (c) 2013 Sergey Potapov. See LICENSE.txt for further details.