# Unit test for f()=print("something")
**URL:** https://discourse.julialang.org/t/unit-test-for-f-print-something/9876
**Category:** General Usage
**Created:** [March 22, 2018, 3:39am UTC](https://discourse.julialang.org/t/unit-test-for-f-print-something/9876 "2018-03-22T03:39:28Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@jandehaan](https://discourse.julialang.org/u/jandehaan)
#### Post date: [March 22, 2018, 3:39am UTC](https://discourse.julialang.org/t/unit-test-for-f-print-something/9876/1 "2018-03-22T03:39:28Z")
I would like to write unit tests like the following
```julia
f() = print("something")
@test print_to_string(f) == "something"
```
I’m not able to successfully define the function `print_to_string`. I’m getting lost while searching the internet including this Julia discourse forum for solutions. There is talk of “redirecting\_stdout”, “blocking”, “tasks”, “channels”, etc.
Any suggestions for how to define the function `print_to_string`?
---
### Author:  [@rdeits](https://discourse.julialang.org/u/rdeits)
#### Post date: [March 22, 2018, 3:55am UTC](https://discourse.julialang.org/t/unit-test-for-f-print-something/9876/2 "2018-03-22T03:55:01Z")
Try `@capture_out` from [https://github.com/JuliaIO/Suppressor.jl](https://github.com/JuliaIO/Suppressor.jl)
---
### Author:  [@jandehaan](https://discourse.julialang.org/u/jandehaan)
#### Post date: [March 22, 2018, 4:02am UTC](https://discourse.julialang.org/t/unit-test-for-f-print-something/9876/3 "2018-03-22T04:02:00Z")
I looked at that, but I couldn’t figure out what `block` should contain, or how to get my hands on the thing that is being captured. I got the impression that the output of `block` was simply being thrown in the “bit bucket”, that is, being suppressed. Did I misunderstand how @capture\_out actually works?
---
### Author:  [@rdeits](https://discourse.julialang.org/u/rdeits)
#### Post date: [March 22, 2018, 4:31am UTC](https://discourse.julialang.org/t/unit-test-for-f-print-something/9876/4 "2018-03-22T04:31:23Z")
`@capture_out` just returns whatever was printed by the captured expression, e.g.:
```julia
julia> using Suppressor
julia> f() = print("something")
f (generic function with 1 method)
julia> result = @capture_out f()
"something"
julia> result
"something"
```
---
### Author:  [@jandehaan](https://discourse.julialang.org/u/jandehaan)
#### Post date: [March 22, 2018, 4:42am UTC](https://discourse.julialang.org/t/unit-test-for-f-print-something/9876/5 "2018-03-22T04:42:14Z")
aha. Thank you.
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [March 22, 2018, 7:29am UTC](https://discourse.julialang.org/t/unit-test-for-f-print-something/9876/6 "2018-03-22T07:29:52Z")
Also, if you are testing `show` methods, `repr` may be useful.
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [March 22, 2018, 12:14pm UTC](https://discourse.julialang.org/t/unit-test-for-f-print-something/9876/7 "2018-03-22T12:14:20Z")
You can look at the [`@test_warn` macro in Base](https://github.com/JuliaLang/julia/blob/33fdd327728894d4987005322b121282bf8f1b99/stdlib/Test/src/Test.jl#L541-L568) and do something similar except for stdout instead of stderr.
Alternatively, consider whether it makes sense to pass an `io` argument to your function(s) so that you can direct your output to an arbitrary stream.
---
### Author:  [@jandehaan](https://discourse.julialang.org/u/jandehaan)
#### Post date: [March 23, 2018, 12:37am UTC](https://discourse.julialang.org/t/unit-test-for-f-print-something/9876/8 "2018-03-23T00:37:22Z")
Update for those who run into similar issues:
```julia
julia> versioninfo()
Julia Version 0.6.2
Commit d386e40c17* (2017-12-13 18:08 UTC)
Platform Info:
OS: Windows (x86_64-w64-mingw32)
```
```julia
julia> @test_warn "something" error("something")
unlink: resource busy or locked (EBUSY)
```
This is caused by the `rm(fname, force=true)` statement in macro `test_warn`. A web search tells me that it happens on Windows 10 and is either caused by the anti-virus program or by not running Jupyter as adminstrator. I don’t want to turn off the anti-virus program nor do I want to run Julia as `administrator`
Therefore I tried
```julia
julia> using Suppressor
julia> f() = print("something")
julia> result = @capture_out f()
MethodError: no method matching redirect_stdout(::IJulia.IJuliaStdio{Base.PipeEndpoint})
```
No good either. So the solution that works for me is the following.
```julia
print_to_string(f::Function, args...) = begin
io = IOBuffer()
f(io,args...)
String(take!(io))
end
f(io::IO) = print(io, "something")
julia> print_to_string(f) == "something"
true
```
---
### Author:  [@rdeits](https://discourse.julialang.org/u/rdeits)
#### Post date: [March 23, 2018, 4:05am UTC](https://discourse.julialang.org/t/unit-test-for-f-print-something/9876/9 "2018-03-23T04:05:19Z")
The issue you’re seeing with Suppressor on IJulia should hopefully be resolved by [https://github.com/JuliaLang/IJulia.jl/pull/641](https://github.com/JuliaLang/IJulia.jl/pull/641)