Clojure: how to know if a function (test-vars) printed something to out?

Question

I am looking for a way to determine if a function printed (using println or anything similar).

What I am trying to determine is if test-vars printed something or not.

The issue with this function is that it doesn't return anything, but only print things when errors got issues, but I want to know if it succeeded or not and the only way I see to know it is to check if it printed something or not.

Even better would be a way to "intercept" what is going to out and to put it in a variable.

After the comments from @lee and @leetwinski I tried the following:

(deftest test-1
  (testing
  (is (= 1 2))))

Then I tried to run that test using:

(let [printed (with-out-str (test-vars [#'test-1]))]
    (print (str "Printed size: " (count printed))))

But here is what I got:

FAIL in (test-1) (form-init505699766276988020.clj:103)
expected: (= 1 2)
  actual: (not (= 1 2))
Printed size: 0
nil

So, what test-vars outputs has been outputted anyway. Then printed was empty.

leetwinski · Accepted Answer · 2016-05-27 15:07:59Z

1

in addition to @Lee's answer: in general you could just rebind *out* to any other writer you want:

temporary binding:

user> (binding [*out* (java.io.StringWriter.)]
        (println 101)
        (str *out*))
"101\n"

is just an analogue to with-out-str

thread-local binding:

user> (set! *out* (java.io.StringWriter.))
#object[java.io.StringWriter 0x575e6773 ""]

user> (println 123)
nil

user> (str *out*)
"123\n"

here we rebind the *out*, for the current thread (and for all the threads, spawned from it, AFAIK).

answered May 27, 2016 at 15:07

leetwinski

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2 Comments

Neoasimov Over a year ago

Great thanks @leetwinski & @lee, not sure why I didn't thought about with-out-str. It works, but not with test-vars. I am looking at the clojure.test code and I am not sure why. I think it has something to do with *test-out* but I am not sure what. Any ideas?

Neoasimov Over a year ago

You had it almost right, instead of *out* it should have been *test-out* which is a different one used by tests. Otherwise, the method was exactly the one to use. Thanks!

Lee · Accepted Answer · 2016-05-27 14:09:44Z

1

You can use with-out-str which intercepts printed values and collects them into a string:

(let [printed (with-out-str (test-vars))]
    ...)

answered May 27, 2016 at 14:09

Lee

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