README.md

ShowMethodTesting.jl

This is a simple package to help testing Base.show methods defined for custom types. The package exports a single function parse_show which, receiving an instance of a structure, will parse the show output of that structure such that:

• Numbers are isolated from other characters (except dots), and compared up to a precision defined by the user (by default equality for integers and rtol=1e-3 for floats).
• Paths are identified and compared only for their last entry (the file name or last directory entry).
• Arrays, identified by braces, are simplified and only the first and last elements are compared, to avoid errors associated to the number of elements printed.

The function parse_show returns then a custom object which can be compared with isapprox to the string copy/pasted from the show of the custom type.

The parse_show function:

parse_show(x; vector_simplify=true, repl=())

Parse the output of show to a ParsedShow object, which can be compared with isapprox (≈), to other ParsedShow objects or to strings.

Arguments

• x: object to parse
• vector_simplify: if true, only the first and last elements of arrays are kept
• repl: container with custom replacements to be made before parsing. The replacements must be defined as a list of Pair(s), and are applied from left to right in the case of ordered collections. (example: repl=["new" => "old", "bad" => "good"]).

Comparison arguments for isapprox

The isapprox function comparing ParsedShow objects (between each other, or to strings) has the following keyword arguments:

• float_match: function to compare two floats
• int_match: function to compare two integers
• path_match: function to compare two paths
• assertion_error: if true, throws an AssertionError if the comparison fails

Example

julia> using ShowMethodTesting

julia> struct A
           x::Int
           path::String
           vec::Vector{Float64}
       end

julia> Base.show(io::IO, ::MIME"text/plain", a::A) = print(io, "Object with Int(\$(a.x)), \$(a.path) and \$(a.vec)")

julia> a = A(1, "/usr/bin/bash", [1.0, π, 7.5, √2])
Object with Int(1), /usr/bin/bash and [1.0, 3.141592653589793, 7.5, 1.4142135623730951]

julia> parse_show(a) ≈ "Object with Int(1), /usr/bin/bash and [1.0, 3.1415, 7.5, 1.4142]"
true

julia> isapprox(parse_show(a), "Object with Int(1), /usr/bin/bash and [1.0, 3.1415, 7.5, 1.4142]"; float_match = (x,y) -> x == y, assertion_error=false)
false

Note that in the last line we have set the comparison function for floats to be exact, which is why the comparison fails. And the assertion_error is set to false, so the function returns false instead of throwing an error.

The utility of throwing an error is that the error message contains the comparison that caused the failure. For example, here we modified Int(1) to Int(2) in the expected output:

julia> parse_show(a) ≈ "Object with Int(2), /usr/bin/bash and [1.0, 3.1415, 7.5, 1.4142]"
ERROR: AssertionError: 


    show method comparison failed for 1 (Int64) == 2 (Int64)


Stacktrace:
...