assert/require.Len doesn't print anything if the slice is too long

Hey there, I encountered this when attempting to use require.Len as shown below.

package tester_test

import (
	"testing"

	"github.com/stretchr/testify/require"
)

func TestVeryLongSlice(t *testing.T) {
	var bigSlice []string
	for i := 0; i < 20000; i++ {
		bigSlice = append(bigSlice, "hello")
	}

	require.Len(t, bigSlice, 10)
}

Output of test:

=== RUN   TestVeryLongSlice
    tester_test.go:15:
                Error Trace:    C:/Users/Fots/source/tester/tester/tester_test.go:15
                Error:
                Test:           TestVeryLongSlice
--- FAIL: TestVeryLongSlice (0.00s)
FAIL
FAIL    github.com/fgimian/tester/tester        0.150s
FAIL

I haven't quite figured out why this happens, although I can imagine the output would be impractically long anyway. Worst of all, the main assertion message is not displayed which is quite confusing (especially to someone new using the library, like me 😄).

Personally, I think that it would be best not to display the slice at all for the Len function, and instead just display:

Should have X item(s), but has Y

However, I suspect my view on that may not be what some people want. The reason I think it is unwise to print the slice in any form, is that typically when many people are testing for the length of a slice, they may be comparing to a relatively large slice of items (e.g. the output of a mocked API call or similar).

An additional point that I believe has been discussed before is the inconsistency with argument ordering for the Len function which takes the actual first and expected second.

Cheers
Fotis

The easiest workaround for now seems to be:

package tester_test

import (
	"testing"

	"github.com/stretchr/testify/require"
)

func TestVeryLongSlice(t *testing.T) {
	var bigSlice []string
	for i := 0; i < 20000; i++ {
		bigSlice = append(bigSlice, "hello")
	}

	expected := 10
	require.Len(
		t,
		bigSlice,
		expected,
		"Should have %d item(s), but has %d",
		expected,
		len(bigSlice),
	)
}

We use a scanner to get the text which should go there line by line (it can be multiline) to indent each line until after the field name:

testify/assert/assertions.go

Lines 304 to 313 in 7caada5

    
           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") 
        
           	} 
        
           	outBuf.WriteString(scanner.Text()) 
        
           } 
        
           return outBuf.String()

This mishandles the scanner.Scan() call. If scanner.Scan() returns false then you need to check scanner.Err() in case it isn't io.EOF. If we did that we'd have found it is bufio.Scanner: token too long.

We need to think of a thing to do about very long lines in this code. We should also probably not produce such a long line from assert.Len, maybe we should truncate after a sane number of elements?

We use a scanner to get the text which should go there line by line (it can be multiline) to indent each line until after the field name:

testify/assert/assertions.go

Lines 304 to 313 in 7caada5

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")

}

outBuf.WriteString(scanner.Text())

}

return outBuf.String()

This mishandles the scanner.Scan() call. If scanner.Scan() returns false then you need to check scanner.Err() in case it isn't io.EOF. If we did that we'd have found it is bufio.Scanner: token too long.

We need to think of a thing to do about very long lines in this code. We should also probably not produce such a long line from assert.Len, maybe we should truncate after a sane number of elements?

That sounds good to me. Personally I'd even be happy for the contents of the slice not to be printed at all, but I suppose this output can be useful for shorter slices when troubleshooting why a test is failing.

Cheers
Fotis

Hello
For the argument ordering, you can enable https://github.com/Antonboom/testifylint via golangci

Hello For the argument ordering, you can enable https://github.com/Antonboom/testifylint via golangci

Thanks, yep I'm using this already, but I think my point was more around the inconsistency of the API and how that particular function feels odd in the context of the rest.

Of course I appreciate that this would be a breaking change but just thought I'd bring it up for consideration in case a major new version of testify was being planned.

Cheers
Fotis

The argument order is covered by #146. Let's keep this issue about the missing message.

assert: handle TokenTooLong error scenario while formatting assertion…

This mishandles the scanner.Scan() call. If scanner.Scan() returns false then you need to check scanner.Err() in case it isn't io.EOF. If we did that we'd have found it is bufio.Scanner: token too long.

I've raised a PR that will make sure that the TokenTooLong never happens.

We should also probably not produce such a long line from assert.Len, maybe we should truncate after a sane number of elements?

Agreed. But regardless of the call we take about displaying long messages, I think this fix will be useful.

I hit this recently and debugged it to the same root cause. For example the following test:

func TestContainsLonglineRepro(t *testing.T) {
	for _, numEntries := range []int{100, 10000} {
		t.Run(fmt.Sprintf("numEntries=%d", numEntries), func(t *testing.T) {
			m := map[string]string{}
			for i := 0; i < numEntries; i++ {
				m[fmt.Sprintf("key%d", i)] = fmt.Sprintf("value%d", i)
			}
			require.Contains(t, m, "not-here")
		})
	}
}

Produces:

--- FAIL: TestContainsLonglineRepro (0.02s)
    --- FAIL: TestContainsLonglineRepro/numEntries=100 (0.00s)
        main_test.go:17:
                Error Trace:    /home/mike/testify-tests/main_test.go:17
                Error:          map[string]string{"key0":"value0", "key1":"value1", "key10":"value10", "key11":"value11", "key12":"value12", "key13":"value13", "key14":"value14", "key15":"value15", "key16":"value16", "key17":"value17", "key18":"value18", "key19":"value19", "key2":"value2", "key20":"value20", "key21":"value21", "key22":"value22", "key23":"value23", "key24":"value24", "key25":"value25", "key26":"value26", "key27":"value27", "key28":"value28", "key29":"value29", "key3":"value3", "key30":"value30", "key31":"value31", "key32":"value32", "key33":"value33", "key34":"value34", "key35":"value35", "key36":"value36", "key37":"value37", "key38":"value38", "key39":"value39", "key4":"value4", "key40":"value40", "key41":"value41", "key42":"value42", "key43":"value43", "key44":"value44", "key45":"value45", "key46":"value46", "key47":"value47", "key48":"value48", "key49":"value49", "key5":"value5", "key50":"value50", "key51":"value51", "key52":"value52", "key53":"value53", "key54":"value54", "key55":"value55", "key56":"value56", "key57":"value57", "key58":"value58", "key59":"value59", "key6":"value6", "key60":"value60", "key61":"value61", "key62":"value62", "key63":"value63", "key64":"value64", "key65":"value65", "key66":"value66", "key67":"value67", "key68":"value68", "key69":"value69", "key7":"value7", "key70":"value70", "key71":"value71", "key72":"value72", "key73":"value73", "key74":"value74", "key75":"value75", "key76":"value76", "key77":"value77", "key78":"value78", "key79":"value79", "key8":"value8", "key80":"value80", "key81":"value81", "key82":"value82", "key83":"value83", "key84":"value84", "key85":"value85", "key86":"value86", "key87":"value87", "key88":"value88", "key89":"value89", "key9":"value9", "key90":"value90", "key91":"value91", "key92":"value92", "key93":"value93", "key94":"value94", "key95":"value95", "key96":"value96", "key97":"value97", "key98":"value98", "key99":"value99"} does not contain "not-here"
                Test:           TestContainsLonglineRepro/numEntries=100
    --- FAIL: TestContainsLonglineRepro/numEntries=10000 (0.02s)
        main_test.go:17:
                Error Trace:    /home/mike/testify-tests/main_test.go:17
                Error:
                Test:           TestContainsLonglineRepro/numEntries=10000
FAIL
FAIL    testify-tests   0.025s

I think the idea of truncating the error after a certain length (1k?, 4k?) seems reasonable, and avoids spamming a huge amount of unnecessary output while making sure that the error message is useful.

Looking at the usage code it might be a bit awkward to do this though (https://github.com/stretchr/testify/blob/master/assert/assertions.go#L938). I guess everywhere messages are formatted would need to be modified with a call to a function that would format and then truncate.

I'd be happy to put together a PR if there is agreement on the right approach.

Personally, I think that it would be best not to display the slice at all for the Len function, and instead just display:\n\nShould have X item(s), but has Y\nHowever, I suspect my view on that may not be what some people want. The reason I think it is unwise to print the slice in any form, is that typically when many people are testing for the length of a slice, they may be comparing to a relatively large slice of items (e.g. the output of a mocked API call or similar).

I totally agree with you

One naive solution could be to check the length of the sprinted value, and truncate it after let's say an arbitrary value of 20-30 characters. Something with a trailing … plus tge number of items.

If the slice is short and simple, such as a slice 2-3 int/string/bool, the message displayed will be the same as now. The error message will be meaningful.

In other cases, where the message is currently useless, it would be better than nothing.

Any thoughts guys?

One naive solution could be to check the length of the sprinted value, and truncate it after let's say an arbitrary value of 20-30 characters. Something with a trailing … plus tge number of items.

If the slice is short and simple, such as a slice 2-3 int/string/bool, the message displayed will be the same as now. The error message will be meaningful.

In other cases, where the message is currently useless, it would be better than nothing.

Any thoughts guys?

A very rough draft, but potentially something like:

func Contains(t TestingT, s, contains interface{}, msgAndArgs ...interface{}) bool {
	if h, ok := t.(tHelper); ok {
		h.Helper()
	}

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

Replaced with

func truncatingSprintf(format string, a ...any) string {
	if !strings.HasPrefix(format, "%#v") {
		panic("TruncatingSprintf format argument must begin with '%#v'")
	}
	result := fmt.Sprintf(format, a...)
	if len(result) < 1024 {
		return result
	}
	truncatedFormat := "object of type %T" + format[3:]
	return fmt.Sprintf(truncatedFormat, len(result))
}

func Contains(t TestingT, s, contains interface{}, msgAndArgs ...interface{}) bool {
	if h, ok := t.(tHelper); ok {
		h.Helper()
	}

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

Except we were talking about "Len", and not "Contains", yes it's exactly what I'm thinking about

Should I assume there is the same issue (dumping a slice) with Contains?

Except we were talking about "Len", and not "Contains", yes it's exactly what I'm thinking about

Should I assume there is the same issue (dumping a slice) with Contains?

Ah yes, I guess there is the same issue for any function which operates on containers (at least Len and Contains, but perhaps others?)

1024 seems a very high value.

It's OK for the example reported here, but having 1024 characters dumped on screen, which means 13 screens of text (assuming 80x25 terminal)

maybe 1024 is fine, but then we could simply add an "\n" in the formatted string. This way the message Len expected actual would be visible

Let's wait for other people feedbacks. But, yes anything like that would be better than current messages

In other code I have written is was presented a little differently:

return fmt.Errorf("wrong result length: wanted %d, got %d: %s>", want, got, f.snippet(v))

Where snippet() was:

// snippet produces a short snippet string of an arbitrary value.                                                                                                                
func (f Formatter) snippet(v any) string {
    const snipLen = 16

    snip := fmt.Sprintf("%#v", v)
    if len(snip) > snipLen {
        // TODO: maybe add a "... %d more"
        snip = snip[:snipLen]
    }   
    return snip
}

They key parts being:

The snippet goes LAST, not first.
The error condition goes FIRST, not last
The snippet is very small - 16 chars, or 32, but not 1024.
You can call snippet() on anything and get an abbreviated snippet.

I'm unsure there will be an agreement or consensus on inverting the message and the truncated string. But even if not inverted, I'm OK with it as long we keep the truncated string short so <= 32

Instead of

assert.Len(t, slice, N)

I'm simply using

assert.Equal(t, N, len(slice))

If I'm asserting the length of a slice, it's precisely because I don't care about the slice items. Otherwise, I would have used assert.Equal() instead. So for me, a good solution would also be simply showing the length mismatch on failure, never the items of the slice.

There actually is already a truncatingSprintf function in testify used by Equal. It's called truncatingFormat and it comes with an existing decision on what the limit should be. I've opened a PR to use it in other relevant assertions. If we feel the limit should be smaller than the technical maximum then we can address that in another PR.

Closed by #1646

Lines should now be not too long that they can't be printed to the terminal. #1801 will choose a lower limit where they are not too big to be useful to humans.

Navigation Menu

Repository navigation

assert/require.Len doesn't print anything if the slice is too long #1525

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

	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")
	}
	outBuf.WriteString(scanner.Text())
	}

	return outBuf.String()

assert/require.Len doesn't print anything if the slice is too long #1525

Description

Activity

fgimian commented on Jan 23, 2024

brackendawson commented on Feb 21, 2024

fgimian commented on Feb 21, 2024

hendrywiranto commented on Feb 22, 2024

fgimian commented on Feb 22, 2024

brackendawson commented on Feb 23, 2024

arjunmahishi commented on Mar 3, 2024

mirober commented on Jul 6, 2024

ccoVeille commented on Jul 11, 2024

ccoVeille commented on Jul 11, 2024

mirober commented on Jul 11, 2024

ccoVeille commented on Jul 11, 2024

mirober commented on Jul 11, 2024

ccoVeille commented on Jul 11, 2024

ccoVeille commented on Jul 11, 2024

thockin commented on Jul 11, 2024

ccoVeille commented on Jul 13, 2024

jdemeyer commented on Sep 27, 2024

brackendawson commented on Oct 3, 2024

brackendawson commented on Sep 19, 2025

brackendawson commented on Sep 19, 2025

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions