PyObjC is the way to go:

#!/usr/bin/python

from AppKit import NSPasteboard, NSStringPboardType

pb = NSPasteboard.generalPasteboard()
pbstring = pb.stringForType_(NSStringPboardType)
print u"Pastboard string: %s".encode("utf-8") % repr(pbstring)

This only supports text and will return None otherwise. You can extend it to support other data types as well, see NSPastboard Class Reference.

How to write a Unicode string to the Mac clipboard:

import subprocess

def write_to_clipboard(output):
    process = subprocess.Popen(
        'pbcopy', env={'LANG': 'en_US.UTF-8'}, stdin=subprocess.PIPE)
    process.communicate(output.encode('utf-8'))

How to read a Unicode string from the Mac clipboard:

import subprocess

def read_from_clipboard():
    return subprocess.check_output(
        'pbpaste', env={'LANG': 'en_US.UTF-8'}).decode('utf-8')

Works on both Python 2.7 and Python 3.4.

2021 Update: If you need to be able to read the clipboard on other operating systems and not just Mac and are okay with adding an external library, pyperclip also seems to work well. I tested it on Mac with Unicode text:

python -m pip install pyperclip
python -c 'import pyperclip; pyperclip.copy("私はDavid!")'  # copy
python -c 'import pyperclip; print(repr(pyperclip.paste()))'  # paste

This little piece of ugliness works and loads "image.jpg" onto the clipboard...

#!/usr/bin/env python3

import subprocess

subprocess.run(["osascript", "-e", 'set the clipboard to (read (POSIX file "image.jpg") as JPEG picture)'])

You can use the module called win32clipboard, which is part of pywin32.

Here is an example that first sets the clipboard data then gets it:

import win32clipboard

# set clipboard data
win32clipboard.OpenClipboard()
win32clipboard.EmptyClipboard()
win32clipboard.SetClipboardText('testing 123')
win32clipboard.CloseClipboard()

# get clipboard data
win32clipboard.OpenClipboard()
data = win32clipboard.GetClipboardData()
win32clipboard.CloseClipboard()
print data

An important reminder from the documentation:

When the window has finished examining or changing the clipboard, close the clipboard by calling CloseClipboard. This enables other windows to access the clipboard. Do not place an object on the clipboard after calling CloseClipboard.

I'm not sure if this will solve for pasting into an email, but I came across this question while looking for how to copy stuff to the clipboard in macOS and have since found a solution that works for me. I figured I'd post it in case anyone else happens across this post with similar needs.

As mentioned in this question's comments, I came across Pyperclip. However I don't want any external dependencies. So, I took a peak at how Pyperclip implemented copy/paste and it's relatively simple. See here.

The short of it is this:

#!/usr/bin/env python3

import subprocess

# copy
def pbcopy(txt):
    task = subprocess.Popen(
        ['pbcopy'],
        stdin=subprocess.PIPE,
        close_fds=True
    )
    task.communicate(input=txt.encode('utf-8'))

# paste
def pbpaste():
    task = subprocess.Popen(
        ['pbpaste'],
        stdout=subprocess.PIPE,
        close_fds=True
    )
    stdout, stderr = task.communicate()
    return(stdout.decode('utf-8'))

Hopefully this solves @MSawers question and if not hopefully someone else will find this useful.