cpython/InternalDocs/locations.md

2.3 KiB

Locations table

The co_linetable bytes object of code objects contains a compact representation of the source code positions of instructions, which are returned by the co_positions() iterator.

co_linetable consists of a sequence of location entries. Each entry starts with a byte with the most significant bit set, followed by zero or more bytes with most significant bit unset.

Each entry contains the following information:

  • The number of code units covered by this entry (length)
  • The start line
  • The end line
  • The start column
  • The end column

The first byte has the following format:

Bit 7 Bits 3-6 Bits 0-2
1 Code Length (in code units) - 1

The codes are enumerated in the _PyCodeLocationInfoKind enum.

Variable length integer encodings

Integers are often encoded using a variable length integer encoding

Unsigned integers (varint)

Unsigned integers are encoded in 6 bit chunks, least significant first. Each chunk but the last has bit 6 set. For example:

  • 63 is encoded as 0x3f
  • 200 is encoded as 0x48, 0x03

Signed integers (svarint)

Signed integers are encoded by converting them to unsigned integers, using the following function:

def convert(s):
    if s < 0:
        return ((-s)<<1) | 1
    else:
        return (s<<1)

Location entries

The meaning of the codes and the following bytes are as follows:

Code Meaning Start line End line Start column End column
0-9 Short form Δ 0 Δ 0 See below See below
10-12 One line form Δ (code - 10) Δ 0 unsigned byte unsigned byte
13 No column info Δ svarint Δ 0 None None
14 Long form Δ svarint Δ varint varint varint
15 No location None None None None

The Δ means the value is encoded as a delta from another value:

  • Start line: Delta from the previous start line, or co_firstlineno for the first entry.
  • End line: Delta from the start line

The short forms

Codes 0-9 are the short forms. The short form consists of two bytes, the second byte holding additional column information. The code is the start column divided by 8 (and rounded down).

  • Start column: (code*8) + ((second_byte>>4)&7)
  • End column: start_column + (second_byte&15)