Can someone describe the details of how Python loads modules into
memory? I assume once the .py file is compiled to .pyc that it is
mmap'ed in. But that assumption is very naive. Maybe it uses an
anonymous mapping? Maybe it does other special magic? This is all
very alien to me, so if someone could explain it in terms that a
person who never usually worries about memory could understand, that
would be much appreciated.
There is no magic whatsoever. Python opens a sequential file descriptor
for the .pyc file, and then reads it in small chunks, "unmarshalling"
it (indeed, the marshal module is used to restore Python objects).
The marshal format is an object serialization in a type-value encoding
(sometimes type-length-value), with type codes for:
- None, True, False
- 32-bit ints, 64-bit ints (unmarshalled into int/long)
- floats, complex
- arbitrary-sized longs
- strings, unicode
- tuples (length + marshal data of values)
- code objects
- a few others
Result of unmarshalling is typically a code object.
Follow up: is this process different if the modules are loaded from a
No; it uncompresses into memory, and then unmarshals from there (
compressed block for compressed block)
If there is a link that covers this info, that'd be great too.
See the description of the marshal module.