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Merged
merged 3 commits into from
Sep 19, 2025
Merged

assert: truncate very long objects in test failure messages #1646

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1eb5fc0
Truncate very long objects in test failure messages
brackendawson Oct 3, 2024
28dac59
Don't shadow builtin max
brackendawson Sep 18, 2025
d5be414
Don't change the format of failure messages when fixing 1525
brackendawson Sep 18, 2025
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90 changes: 46 additions & 44 deletions assert/assertions.go
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Original file line number Diff line number Diff line change
Expand Up @@ -325,13 +325,15 @@ func messageFromMsgAndArgs(msgAndArgs ...interface{}) string {
func indentMessageLines(message string, longestLabelLen int) string {
outBuf := new(bytes.Buffer)

for i, scanner := 0, bufio.NewScanner(strings.NewReader(message)); scanner.Scan(); i++ {
// no need to align first line because it starts at the correct location (after the label)
if i != 0 {
// append alignLen+1 spaces to align with "{{longestLabel}}:" before adding tab
outBuf.WriteString("\n\t" + strings.Repeat(" ", longestLabelLen+1) + "\t")
scanner := bufio.NewScanner(strings.NewReader(message))
for firstLine := true; scanner.Scan(); firstLine = false {
if !firstLine {
fmt.Fprint(outBuf, "\n\t"+strings.Repeat(" ", longestLabelLen+1)+"\t")
}
outBuf.WriteString(scanner.Text())
fmt.Fprint(outBuf, scanner.Text())
}
if err := scanner.Err(); err != nil {
return fmt.Sprintf("cannot display message: %s", err)
}

return outBuf.String()
Expand Down Expand Up @@ -544,9 +546,8 @@ func Same(t TestingT, expected, actual interface{}, msgAndArgs ...interface{}) b
if !same {
// both are pointers but not the same type & pointing to the same address
return Fail(t, fmt.Sprintf("Not same: \n"+
"expected: %#[1]v (%[1]T)(%[1]p)\n"+
"actual : %#[2]v (%[2]T)(%[2]p)",
expected, actual), msgAndArgs...)
"expected: %[2]s (%[1]T)(%[1]p)\n"+
"actual : %[4]s (%[3]T)(%[3]p)", expected, truncatingFormat("%#v", expected), actual, truncatingFormat("%#v", actual)), msgAndArgs...)
}

return true
Expand All @@ -571,8 +572,8 @@ func NotSame(t TestingT, expected, actual interface{}, msgAndArgs ...interface{}

if same {
return Fail(t, fmt.Sprintf(
"Expected and actual point to the same object: %p %#[1]v",
expected), msgAndArgs...)
"Expected and actual point to the same object: %p %s",
expected, truncatingFormat("%#v", expected)), msgAndArgs...)
}
return true
}
Expand Down Expand Up @@ -604,25 +605,26 @@ func samePointers(first, second interface{}) (same bool, ok bool) {
// to a type conversion in the Go grammar.
func formatUnequalValues(expected, actual interface{}) (e string, a string) {
if reflect.TypeOf(expected) != reflect.TypeOf(actual) {
return fmt.Sprintf("%T(%s)", expected, truncatingFormat(expected)),
fmt.Sprintf("%T(%s)", actual, truncatingFormat(actual))
return fmt.Sprintf("%T(%s)", expected, truncatingFormat("%#v", expected)),
fmt.Sprintf("%T(%s)", actual, truncatingFormat("%#v", actual))
}
switch expected.(type) {
case time.Duration:
return fmt.Sprintf("%v", expected), fmt.Sprintf("%v", actual)
}
return truncatingFormat(expected), truncatingFormat(actual)
return truncatingFormat("%#v", expected), truncatingFormat("%#v", actual)
}

// truncatingFormat formats the data and truncates it if it's too long.
//
// This helps keep formatted error messages lines from exceeding the
// bufio.MaxScanTokenSize max line length that the go testing framework imposes.
func truncatingFormat(data interface{}) string {
value := fmt.Sprintf("%#v", data)
max := bufio.MaxScanTokenSize - 100 // Give us some space the type info too if needed.
if len(value) > max {
value = value[0:max] + "<... truncated>"
func truncatingFormat(format string, data interface{}) string {
value := fmt.Sprintf(format, data)
// Give us space for two truncated objects and the surrounding sentence.
maxMessageSize := bufio.MaxScanTokenSize/2 - 100
if len(value) > maxMessageSize {
value = value[0:maxMessageSize] + "<... truncated>"
}
return value
}
Expand Down Expand Up @@ -743,7 +745,7 @@ func Nil(t TestingT, object interface{}, msgAndArgs ...interface{}) bool {
if h, ok := t.(tHelper); ok {
h.Helper()
}
return Fail(t, fmt.Sprintf("Expected nil, but got: %#v", object), msgAndArgs...)
return Fail(t, fmt.Sprintf("Expected nil, but got: %s", truncatingFormat("%#v", object)), msgAndArgs...)
}

// isEmpty gets whether the specified object is considered empty or not.
Expand Down Expand Up @@ -793,7 +795,7 @@ func Empty(t TestingT, object interface{}, msgAndArgs ...interface{}) bool {
if h, ok := t.(tHelper); ok {
h.Helper()
}
Fail(t, fmt.Sprintf("Should be empty, but was %v", object), msgAndArgs...)
Fail(t, fmt.Sprintf("Should be empty, but was %s", truncatingFormat("%v", object)), msgAndArgs...)
}

return pass
Expand Down Expand Up @@ -836,11 +838,11 @@ func Len(t TestingT, object interface{}, length int, msgAndArgs ...interface{})
}
l, ok := getLen(object)
if !ok {
return Fail(t, fmt.Sprintf("\"%v\" could not be applied builtin len()", object), msgAndArgs...)
return Fail(t, fmt.Sprintf("%q could not be applied builtin len()", truncatingFormat("%v", object)), msgAndArgs...)
}

if l != length {
return Fail(t, fmt.Sprintf("\"%v\" should have %d item(s), but has %d", object, length, l), msgAndArgs...)
return Fail(t, fmt.Sprintf("%q should have %d item(s), but has %d", truncatingFormat("%v", object), length, l), msgAndArgs...)
}
return true
}
Expand Down Expand Up @@ -889,7 +891,7 @@ func NotEqual(t TestingT, expected, actual interface{}, msgAndArgs ...interface{
}

if ObjectsAreEqual(expected, actual) {
return Fail(t, fmt.Sprintf("Should not be: %#v\n", actual), msgAndArgs...)
return Fail(t, fmt.Sprintf("Should not be: %s\n", truncatingFormat("%#v", actual)), msgAndArgs...)
}

return true
Expand All @@ -904,7 +906,7 @@ func NotEqualValues(t TestingT, expected, actual interface{}, msgAndArgs ...inte
}

if ObjectsAreEqualValues(expected, actual) {
return Fail(t, fmt.Sprintf("Should not be: %#v\n", actual), msgAndArgs...)
return Fail(t, fmt.Sprintf("Should not be: %s\n", truncatingFormat("%#v", actual)), msgAndArgs...)
}

return true
Expand Down Expand Up @@ -964,10 +966,10 @@ func Contains(t TestingT, s, contains interface{}, msgAndArgs ...interface{}) bo

ok, found := containsElement(s, contains)
if !ok {
return Fail(t, fmt.Sprintf("%#v could not be applied builtin len()", s), msgAndArgs...)
return Fail(t, fmt.Sprintf("%s could not be applied builtin len()", truncatingFormat("%#v", s)), msgAndArgs...)
}
if !found {
return Fail(t, fmt.Sprintf("%#v does not contain %#v", s, contains), msgAndArgs...)
return Fail(t, fmt.Sprintf("%s does not contain %#v", truncatingFormat("%#v", s), contains), msgAndArgs...)
}

return true
Expand All @@ -986,10 +988,10 @@ func NotContains(t TestingT, s, contains interface{}, msgAndArgs ...interface{})

ok, found := containsElement(s, contains)
if !ok {
return Fail(t, fmt.Sprintf("%#v could not be applied builtin len()", s), msgAndArgs...)
return Fail(t, fmt.Sprintf("%s could not be applied builtin len()", truncatingFormat("%#v", s)), msgAndArgs...)
}
if found {
return Fail(t, fmt.Sprintf("%#v should not contain %#v", s, contains), msgAndArgs...)
return Fail(t, fmt.Sprintf("%s should not contain %#v", truncatingFormat("%#v", s), contains), msgAndArgs...)
}

return true
Expand Down Expand Up @@ -1031,10 +1033,10 @@ func Subset(t TestingT, list, subset interface{}, msgAndArgs ...interface{}) (ok
av := actualMap.MapIndex(k)

if !av.IsValid() {
return Fail(t, fmt.Sprintf("%#v does not contain %#v", list, subset), msgAndArgs...)
return Fail(t, fmt.Sprintf("%s does not contain %s", truncatingFormat("%#v", list), truncatingFormat("%#v", subset)), msgAndArgs...)
}
if !ObjectsAreEqual(ev.Interface(), av.Interface()) {
return Fail(t, fmt.Sprintf("%#v does not contain %#v", list, subset), msgAndArgs...)
return Fail(t, fmt.Sprintf("%s does not contain %s", truncatingFormat("%#v", list), truncatingFormat("%#v", subset)), msgAndArgs...)
Comment on lines +1036 to +1039
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I'm a bit worried about these 2

We have 2 list that could be huge that would be truncated here, but displayed on the same line.

Maybe it works because of the max/2-200, but I feel like the line might difficult to read because of the lack of \n that we have for almost all others failures

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@brackendawson brackendawson Sep 18, 2025 •
edited
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I agree about the hard to read part, but that would be a different issue. Again the only aim here is to show strings which previously got swallowed.

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I understand your point. But mine wasn't about readability.

I just wanted to make sure that comparing two slices that very long won't lead to a line that cannot be displayed.

I feel like that tge point of the max/2-200 but I wanted to make sure it would work.

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@brackendawson brackendawson Sep 19, 2025 •
edited
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Yes, that doesn't sit 100% with me either. When we follow this up with #1235 and reduce the max length much further I will be very confident.

}
}

Expand All @@ -1056,7 +1058,7 @@ func Subset(t TestingT, list, subset interface{}, msgAndArgs ...interface{}) (ok
return Fail(t, fmt.Sprintf("%#v could not be applied builtin len()", list), msgAndArgs...)
}
if !found {
return Fail(t, fmt.Sprintf("%#v does not contain %#v", list, element), msgAndArgs...)
return Fail(t, fmt.Sprintf("%s does not contain %#v", truncatingFormat("%#v", list), element), msgAndArgs...)
}
}

Expand Down Expand Up @@ -1106,7 +1108,7 @@ func NotSubset(t TestingT, list, subset interface{}, msgAndArgs ...interface{})
}
}

return Fail(t, fmt.Sprintf("%q is a subset of %q", subset, list), msgAndArgs...)
return Fail(t, fmt.Sprintf("%s is a subset of %s", truncatingFormat("%q", subset), truncatingFormat("%q", list)), msgAndArgs...)
}

subsetList := reflect.ValueOf(subset)
Expand All @@ -1128,7 +1130,7 @@ func NotSubset(t TestingT, list, subset interface{}, msgAndArgs ...interface{})
}
}

return Fail(t, fmt.Sprintf("%q is a subset of %q", subset, list), msgAndArgs...)
return Fail(t, fmt.Sprintf("%s is a subset of %s", truncatingFormat("%q", subset), truncatingFormat("%q", list)), msgAndArgs...)
}

// ElementsMatch asserts that the specified listA(array, slice...) is equal to specified
Expand Down Expand Up @@ -1641,7 +1643,7 @@ func NoError(t TestingT, err error, msgAndArgs ...interface{}) bool {
if h, ok := t.(tHelper); ok {
h.Helper()
}
return Fail(t, fmt.Sprintf("Received unexpected error:\n%+v", err), msgAndArgs...)
return Fail(t, fmt.Sprintf("Received unexpected error:\n%s", truncatingFormat("%+v", err)), msgAndArgs...)
}

return true
Expand Down Expand Up @@ -1680,7 +1682,7 @@ func EqualError(t TestingT, theError error, errString string, msgAndArgs ...inte
if expected != actual {
return Fail(t, fmt.Sprintf("Error message not equal:\n"+
"expected: %q\n"+
"actual : %q", expected, actual), msgAndArgs...)
"actual : %s", expected, truncatingFormat("%q", actual)), msgAndArgs...)
}
return true
}
Expand All @@ -1700,7 +1702,7 @@ func ErrorContains(t TestingT, theError error, contains string, msgAndArgs ...in

actual := theError.Error()
if !strings.Contains(actual, contains) {
return Fail(t, fmt.Sprintf("Error %#v does not contain %#v", actual, contains), msgAndArgs...)
return Fail(t, fmt.Sprintf("Error %s does not contain %#v", truncatingFormat("%#v", actual), contains), msgAndArgs...)
}

return true
Expand Down Expand Up @@ -1766,7 +1768,7 @@ func Zero(t TestingT, i interface{}, msgAndArgs ...interface{}) bool {
h.Helper()
}
if i != nil && !reflect.DeepEqual(i, reflect.Zero(reflect.TypeOf(i)).Interface()) {
return Fail(t, fmt.Sprintf("Should be zero, but was %v", i), msgAndArgs...)
return Fail(t, fmt.Sprintf("Should be zero, but was %s", truncatingFormat("%v", i)), msgAndArgs...)
}
return true
}
Expand Down Expand Up @@ -2206,8 +2208,8 @@ func ErrorIs(t TestingT, err, target error, msgAndArgs ...interface{}) bool {
chain := buildErrorChainString(err, false)

return Fail(t, fmt.Sprintf("Target error should be in err chain:\n"+
"expected: %q\n"+
"in chain: %s", expectedText, chain,
"expected: %s\n"+
"in chain: %s", truncatingFormat("%q", expectedText), truncatingFormat("%s", chain),
), msgAndArgs...)
}

Expand All @@ -2229,8 +2231,8 @@ func NotErrorIs(t TestingT, err, target error, msgAndArgs ...interface{}) bool {
chain := buildErrorChainString(err, false)

return Fail(t, fmt.Sprintf("Target error should not be in err chain:\n"+
"found: %q\n"+
"in chain: %s", expectedText, chain,
"found: %s\n"+
"in chain: %s", truncatingFormat("%q", expectedText), truncatingFormat("%s", chain),
), msgAndArgs...)
}

Expand All @@ -2254,7 +2256,7 @@ func ErrorAs(t TestingT, err error, target interface{}, msgAndArgs ...interface{

return Fail(t, fmt.Sprintf("Should be in error chain:\n"+
"expected: %s\n"+
"in chain: %s", expectedType, chain,
"in chain: %s", expectedType, truncatingFormat("%s", chain),
), msgAndArgs...)
}

Expand All @@ -2272,7 +2274,7 @@ func NotErrorAs(t TestingT, err error, target interface{}, msgAndArgs ...interfa

return Fail(t, fmt.Sprintf("Target error should not be in err chain:\n"+
"found: %s\n"+
"in chain: %s", reflect.TypeOf(target).Elem().String(), chain,
"in chain: %s", reflect.TypeOf(target).Elem().String(), truncatingFormat("%s", chain),
), msgAndArgs...)
}

Expand Down
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