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Add advice on formatting strings using %q #186

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Add advice on formatting strings using `%q`
yuxincs Jun 5, 2023
6b1464c
Auto-update style.md
yuxincs Jun 5, 2023
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1 change: 1 addition & 0 deletions src/SUMMARY.md
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Original file line number Diff line number Diff line change
Expand Up @@ -60,6 +60,7 @@
- [Initializing Maps](map-init.md)
- [Format Strings outside Printf](printf-const.md)
- [Naming Printf-style Functions](printf-name.md)
- [Format Strings Using `%q`](formatting-strings.md)
- Patterns
- [Test Tables](test-table.md)
- [Functional Options](functional-option.md)
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38 changes: 38 additions & 0 deletions src/formatting-strings.md
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@@ -0,0 +1,38 @@
# Format Strings Using `%q`

Whenever formatting messages that contain a string component via `fmt`, use `%q` instead of `%s`. This will wrap the specified string in quotes, helping it stand out from the rest of the error message. More importantly, if the string is empty, it will provide a more helpful error message.
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Whenever formatting messages that contain a string component via fmt, use %q instead of %s.

I think we might want to clarify slightly: it's not necessarily just when a string component in general is used, but more when dynamic strings (incl. user-provided values) could potentially cause interpolation to result in something weird.

Maybe something like:

Whenever dynamic strings - such as user-provided data - are used for formatting,
prefer the placeholder %q over %s.

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[...] helping it stand out from the rest of the error message.

More importantly, if the string is empty, it will provide a more helpful error message.

I'd either generalize these slightly or remove the word "error" (this guidance can apply to more than just errors). Maybe something like:

More importantly, if the string is empty or has its own formatting, the result
will be clearer and more consistent.


<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
fmt.Errrof("file %s not found", filename)
// Prints the following:
// file myfile.go not found
//
// Or if the string is empty:
// file not found
```

</td><td>

```go
fmt.Errrof("file %q not found", filename)
// Prints the following:
// file "myfile.go" not found
//
// Or if the string is empty:
// file "" not found
```

</td></tr>
</tbody></table>

This advice applies more generally to other contexts when reporting user-specified data, such as logging invalid usernames:

```go
log.Printf("User %q does not exist", username)
// User "no_name" does not exist
```
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I would generalize this slightly to something like:

This advice can also apply more generally to other formatting situations
when a dynamic string could confuse or change the formatting or meaning of
surrounding text when interpolated.

Mostly to avoid bucketing "logging" as "formatting" (i.e., structured logging - which is at least as relevant as log.Printf scenarios - doesn't require the same treatment for fields, because they are already discrete values).

I think the log.Printf example is fine to keep, I would just put a good/bad example under the fmt.Errorf examples.

40 changes: 40 additions & 0 deletions style.md
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Expand Up @@ -67,6 +67,7 @@
- [Initializing Maps](#initializing-maps)
- [Format Strings outside Printf](#format-strings-outside-printf)
- [Naming Printf-style Functions](#naming-printf-style-functions)
- [Format Strings Using `%q`](#format-strings-using-q)
- [Patterns](#patterns)
- [Test Tables](#test-tables)
- [Functional Options](#functional-options)
Expand Down Expand Up @@ -3584,6 +3585,45 @@ go vet -printfuncs=wrapf,statusf

See also [go vet: Printf family check](https://kuzminva.wordpress.com/2017/11/07/go-vet-printf-family-check/).

### Format Strings Using `%q`

Whenever formatting messages that contain a string component via `fmt`, use `%q` instead of `%s`. This will wrap the specified string in quotes, helping it stand out from the rest of the error message. More importantly, if the string is empty, it will provide a more helpful error message.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
fmt.Errrof("file %s not found", filename)
// Prints the following:
// file myfile.go not found
//
// Or if the string is empty:
// file not found
```

</td><td>

```go
fmt.Errrof("file %q not found", filename)
// Prints the following:
// file "myfile.go" not found
//
// Or if the string is empty:
// file "" not found
```

</td></tr>
</tbody></table>

This advice applies more generally to other contexts when reporting user-specified data, such as logging invalid usernames:

```go
log.Printf("User %q does not exist", username)
// User "no_name" does not exist
```

## Patterns

### Test Tables
Expand Down