Possible to execute Python bytecode from a script?

Question

Say I have a running CPython session,

Is there a way to run the data (bytes) from a pyc file directly? (without having the data on-disk necessarily, and without having to write a temporary pyc file)

Example script to show a simple use-case:

if foo:
    # Intentionally ambiguous, since the data source
    # is a detail and answers shouldn't depend this detail.
    data = read_data_from_somewhere()
else:
    data = open("bar.pyc", 'rb').read()

assert(type(data) is bytes)

code = bytes_to_code(data)

# call a method from the loaded code
code.call_function()

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Exact use isn't important, but generating code dynamically and copying over a network to execute is one use-case (for the purpose of thinking about this question).

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Here are some example use-cases, which made me curious to know how this can be done:

• Checking Python scripts for malicious code.
  If a single command can access a larger body of code hidden in binary data, what would that command look like?
• Dynamically generate code and cache it for re-use (not necessarily on disk, could use a data-base for example).
• Ability to send pre-compiled byte-code to a process, control an application which embeds Python for eg.

Answer 1

Is there a way to run the data from a pyc file directly?

The compiled code object can be saved using marshal

import marshal
bytes = marshal.dumps(eggs)

the bytes can be converted back to a code object

eggs = marshal.loads(bytes)
exec(eggs)

A pyc file is a marshaled code object with a header

For Python3, the header is 12 bytes which need to be skipped, the remaining data can be read via marshal.loads.

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See Ned Batchelder's blog post:

At the simple level, a .pyc file is a binary file containing only three things:

• A four-byte magic number,
• A four-byte modification timestamp, and
• A marshalled code object.

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Note, the link references Python2, but its almost the same in Python3, the pyc header size is just 12 instead of 8 bytes.

Answer 2

Assuming the platform of the compiled .pyc is correct, you can just import it. So with a file bar.pyc in the python path, the following works even if bar.py does not exist:

import bar
bar.call_function()