Lib/struct.py (part 3)

Source:

cpython 3.14 @ ab2d84fe1023/Lib/struct.py ↗

This annotation covers the module-level struct functions and Struct class caching. See modules_struct3_detail for Struct.pack_into, Struct.unpack_from, and Struct.iter_unpack.

Map

Lines	Symbol	Role
1-80	struct.pack	Pack values into bytes using a format string
81-160	struct.unpack	Unpack bytes into a tuple of values
161-240	struct.calcsize	Return the size of a format string in bytes
241-400	Struct class	Compiled struct format for repeated use

Reading

Module-level functions

# CPython: Lib/struct.py:1 (module-level wrappers)
# The module-level pack/unpack are thin wrappers around Struct objects.
# _struct (C extension) provides the real implementation.
from _struct import (
    pack, pack_into, unpack, unpack_from, iter_unpack,
    calcsize, error, Struct
)

struct.pack(fmt, *values) is equivalent to Struct(fmt).pack(*values). The C extension _struct provides all implementations; the struct Python module just re-exports them. The Struct class caches the compiled format, so repeated calls with the same format are efficient.

Format string reference

Character	C type	Python type	Standard size
b	signed char	int	1
B	unsigned char	int	1
h	short	int	2
H	unsigned short	int	2
i	int	int	4
I	unsigned int	int	4
l	long	int	4
L	unsigned long	int	4
q	long long	int	8
Q	unsigned long long	int	8
f	float	float	4
d	double	float	8
s	char[]	bytes	variable
p	Pascal string	bytes	variable
P	void *	int	native

struct.error

// CPython: Modules/_struct.c:20 struct.error
static PyObject *StructError;  /* exception class */

struct.error is raised for format string errors, size mismatches, and out-of-range values. It's a subclass of Exception. All struct operations raise this rather than ValueError or TypeError.

gopy notes

struct.pack is module/struct.Pack in module/struct/module.go. The format string is compiled to a []formatCode slice. Each format code stores the type char, count, and standard/native size. struct.error is objects.StructError.