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fmt: Precision for strings formatting matches documented neither for %x nor %q #8151

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paulhankin opened this issue Jun 4, 2014 · 5 comments
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@paulhankin
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@paulhankin paulhankin commented Jun 4, 2014

fmt says "For strings, precision is the maximum number of characters to output,
truncating if necessary."

package main

import "fmt"

func main() {
    fmt.Printf("%.1x\n", "hello") // -> 68656c6c6f
    fmt.Printf("%.1q\n", "hello") // -> "h"
}

Which is 10 characters output for the first, and 3 characters output for the second.

Both of these may be working as intended, but I think the fmt module docs need
fixing/clarifying if so.
@ianlancetaylor
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@ianlancetaylor ianlancetaylor commented Jun 4, 2014

Comment 1:

Labels changed: added repo-main, release-go1.4, documentation.

@gopherbot
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@gopherbot gopherbot commented Jun 24, 2014

Comment 3 by MihaiBorobocea:

What is the intended behavior when using precision and %q with strings?
In the example above:
fmt.Printf("%.1q\n", "hello") // -> "h"
The string is truncated to length 1 *before* the quoting.
The logic is straightforward in src/pkg/fmt/format.go::fmt_q():
    truncate()
    quote()
    pad_to_width()
currently at line number:
https://code.google.com/p/go/source/browse/src/pkg/fmt/format.go#325
If the goal here is a strictly quoted string, since quoting can introduce escape
sequences «A\nB» → «A\\\nB» the library can't blindly trim the result and surround
it with quotes: "A\\\" is invalid, and likely dangerous.
If the current behavior (or compromise) is the intended one, maybe mention it in the
docs (an exception applicable only to these “quoting” verbs)? It took me some time
to reason about this and figure out why the printed string is longer than the precision
I'm giving.
I'm leaning towards calling it a bug because, as a user of the fmt package, I expect
.precision to always work the same, without me reasoning about the package's internals.
At least that's what I expected in the absence of disclaimers.
@rsc
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@rsc rsc commented Sep 15, 2014

Comment 4:

Owner changed to @robpike.

Status changed to Accepted.

@gopherbot
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@gopherbot gopherbot commented Sep 24, 2014

Comment 5:

CL https://golang.org/cl/144540044 mentions this issue.
@robpike
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@robpike robpike commented Sep 24, 2014

Comment 6:

This issue was closed by revision 54111a5.

Status changed to Fixed.

@rsc rsc added this to the Go1.4 milestone Apr 14, 2015
@rsc rsc removed the release-go1.4 label Apr 14, 2015
@golang golang locked and limited conversation to collaborators Jun 25, 2016
wheatman added a commit to wheatman/go-akaros that referenced this issue Jun 25, 2018
…slices

Previous behavior was undocumented and inconsistent. Now it is documented
and consistent and measures the input size, since that makes more sense
when talking about %q and %x. For %s the change has no effect.

Fixes golang#8151.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/144540044
wheatman added a commit to wheatman/go-akaros that referenced this issue Jun 26, 2018
…slices

Previous behavior was undocumented and inconsistent. Now it is documented
and consistent and measures the input size, since that makes more sense
when talking about %q and %x. For %s the change has no effect.

Fixes golang#8151.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/144540044
wheatman added a commit to wheatman/go-akaros that referenced this issue Jul 9, 2018
…slices

Previous behavior was undocumented and inconsistent. Now it is documented
and consistent and measures the input size, since that makes more sense
when talking about %q and %x. For %s the change has no effect.

Fixes golang#8151.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/144540044
wheatman added a commit to wheatman/go-akaros that referenced this issue Jul 30, 2018
…slices

Previous behavior was undocumented and inconsistent. Now it is documented
and consistent and measures the input size, since that makes more sense
when talking about %q and %x. For %s the change has no effect.

Fixes golang#8151.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/144540044
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