How to test a function's output (stdout/stderr) in unit tests

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I have a simple function I want to test:

func (t *Thing) print(min_verbosity int, message string) {
    if t.verbosity >= minv {

But how can I test what the function actually sends to standard output? Test::Output does what I want in Perl. I know I could write all my own boilerplate to do the same in Go (as described here):

orig = os.Stdout
r,w,_ = os.Pipe()
thing.print("Some message")
var buf bytes.Buffer
io.Copy(&buf, r)
os.Stdout = orig
if(buf.String() != "Some message") {

But that's a lot of extra work for every single test. I'm hoping there's a more standard way, or perhaps an abstraction library to handle this.

One thing to also remember, there's nothing stopping you from writing functions to avoid the boilerplate.

For example I have a command line app that uses log and I wrote this function:

func captureOutput(f func()) string {
    var buf bytes.Buffer
    return buf.String()

Then used it like this:

output := captureOutput(func() {
assert.Equal("removed certificate\n", output)

Using this assert library:

Algebra II: Functions: Relations and Functions, This video explains how to determine if you have a function. It Duration: 10:02 Posted: Jan 25, 2009 The main function needs to make a call to functiontests to generate a test array, tests. Use localfunctions as the input to functiontests to automatically generate a cell array of function handles to all the local functions in your file. This is a typical main function. function tests = exampleTest tests = functiontests (localfunctions); end

You can do one of two things. The first is to use Examples.

The package also runs and verifies example code. Example functions may include a concluding line comment that begins with "Output:" and is compared with the standard output of the function when the tests are run. (The comparison ignores leading and trailing space.) These are examples of an example:

func ExampleHello() {
        // Output: hello

The second (and more appropriate, IMO) is to use fake functions for your IO. In your code you do:

var myPrint = fmt.Print

func (t *Thing) print(min_verbosity int, message string) {
    if t.verbosity >= minv {
        myPrint(message) // N.B.

And in your tests:

func init() {
    myPrint = fakePrint // fakePrint records everything it's supposed to print.

func Test...

Another option is to use fmt.Fprintf with an io.Writer that is os.Stdout in production code, but may be say bytes.Buffer in tests.

Test of a Function, Suppose there is a function get-data which returns a map of information about the id of the user passed in. Great. You should test that then. Now, SQL Server allows you to run a Select query without a FROM clause. We will use that capability to test our function. Here is an example. select '[email protected]' Now, I want to find the ‘@’ character in the string, so I will use the CharIndex function. Now it is time to test the SQL Server Function: select charindex('@', '[email protected]')

You could consider adding a return statement to your function to return the string that is actually printed out.

func (t *Thing) print(min_verbosity int, message string) string {
    if t.verbosity >= minv {
        return message
    return ""

Now, your test could just check the returned string against an expected string (rather than the print out). Maybe a bit more in-line with Test Driven Development (TDD).

And, in your production code, nothing would need to change, since you don't have to assign the return value of a function if you don't need it.

How should I test the functionality of a function that uses other , A "C" graph would have a single X value that would output 2 Y values. The vertical line test Duration: 2:23 Posted: Dec 25, 2012 To test a function that takes scalar arguments and returns a scalar value: SELECT function_name(parm1, parm2) FROM DUAL; To test a procedure: EXEC procedure_name(parm1, parm2) If the procedure returns a REFCURSORor SYS_REFCURSORby way of an OUTparameter:

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Correct way to test a function, A function like this is called a pure function. Pure functions are the foundation of functional programming, and they all share these three qualities:. The blood sample for liver function tests is usually drawn through a small needle inserted into a vein in the bend of your arm. The needle is attached to a small tube, to collect your blood. You may feel a quick pain as the needle is inserted into your arm and experience some short-term discomfort at the site after the needle is removed.

A Beginner's Guide to Testing Functional JavaScript, Then, when developing a function inside a module, run this function's tests frequently, ideally automatically when you save the code. Always run the full test suite  The purpose of the function is to return a text value for sales status from a given integer. Vary the example by passing integers 1 through 7 to the @Status parameter. USE [AdventureWorks2016CTP3] GO -- Declare a variable to return the results of the function.

  • This is probably what I will do, as it's a more general solution than that provided by @Ainar-G, which requires that I control all of the code which might output something.
  • FYI this is not go-routine safe. A second go-routine could reset log back to os.Stderr while the first go-routing is still logging.
  • Examples are good for when you have deterministic output and fmt.Fprint is good for when you can control the print code (which you can't if it's in an external library or something). I think most of the time one of these things will be true. If there's a case where neither is true an option to read what is written to os.Stdout into a string would be nice.