spew

Package spew implements a deep pretty printer for Go data structures to aid in debugging. A quick overview of the additional features spew provides over the built-in printing facilities for Go data types are as follows: There are two different approaches spew allows for dumping Go data structures: This section demonstrates how to quickly get started with spew. See the sections below for further details on formatting and configuration options. To dump a variable with full newlines, indentation, type, and pointer information use Dump, Fdump, or Sdump: Alternatively, if you would prefer to use format strings with a compacted inline printing style, use the convenience wrappers Printf, Fprintf, etc with %v (most compact), %+v (adds pointer addresses), %#v (adds types), or %#+v (adds types and pointer addresses): Configuration of spew is handled by fields in the ConfigState type. For convenience, all of the top-level functions use a global state available via the spew.Config global. It is also possible to create a ConfigState instance that provides methods equivalent to the top-level functions. This allows concurrent configuration options. See the ConfigState documentation for more details. The following configuration options are available: Indent String to use for each indentation level for Dump functions. It is a single space by default. A popular alternative is "\t". MaxDepth Maximum number of levels to descend into nested data structures. There is no limit by default. DisableMethods Disables invocation of error and Stringer interface methods. Method invocation is enabled by default. DisablePointerMethods Disables invocation of error and Stringer interface methods on types which only accept pointer receivers from non-pointer variables. Pointer method invocation is enabled by default. DisablePointerAddresses DisablePointerAddresses specifies whether to disable the printing of pointer addresses. This is useful when diffing data structures in tests. DisableCapacities DisableCapacities specifies whether to disable the printing of capacities for arrays, slices, maps and channels. This is useful when diffing data structures in tests. ContinueOnMethod Enables recursion into types after invoking error and Stringer interface methods. Recursion after method invocation is disabled by default. SortKeys Specifies map keys should be sorted before being printed. Use this to have a more deterministic, diffable output. Note that only native types (bool, int, uint, floats, uintptr and string) and types which implement error or Stringer interfaces are supported with other types sorted according to the reflect.Value.String() output which guarantees display stability. Natural map order is used by default. SpewKeys Specifies that, as a last resort attempt, map keys should be spewed to strings and sorted by those strings. This is only considered if SortKeys is true. Simply call spew.Dump with a list of variables you want to dump: You may also call spew.Fdump if you would prefer to output to an arbitrary io.Writer. For example, to dump to standard error: A third option is to call spew.Sdump to get the formatted output as a string: See the Dump example for details on the setup of the types and variables being shown here. Byte (and uint8) arrays and slices are displayed uniquely like the hexdump -C command as shown. Spew provides a custom formatter that implements the fmt.Formatter interface so that it integrates cleanly with standard fmt package printing functions. The formatter is useful for inline printing of smaller data types similar to the standard %v format specifier. The custom formatter only responds to the %v (most compact), %+v (adds pointer addresses), %#v (adds types), or %#+v (adds types and pointer addresses) verb combinations. Any other verbs such as %x and %q will be sent to the the standard fmt package for formatting. In addition, the custom formatter ignores the width and precision arguments (however they will still work on the format specifiers not handled by the custom formatter). The simplest way to make use of the spew custom formatter is to call one of the convenience functions such as spew.Printf, spew.Println, or spew.Printf. The functions have syntax you are most likely already familiar with: See the Index for the full list convenience functions. Double pointer to a uint8: Pointer to circular struct with a uint8 field and a pointer to itself: See the Printf example for details on the setup of variables being shown here. Since it is possible for custom Stringer/error interfaces to panic, spew detects them and handles them internally by printing the panic information inline with the output. Since spew is intended to provide deep pretty printing capabilities on structures, it intentionally does not return any errors.
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