gistfile1.py

import os, sys
from subprocess import Popen

# java -cp "%APPDATA%\.minecraft\bin\*" -Djava.library.path="%APPDATA%\.minecraft\bin\natives" net.minecraft.client.Minecraft

appdata = os.path.dirname(os.path.abspath(__file__))

# java -cp ...
args = ['java', '-cp',]

# ... "%APPDATA%\.minecraft\bin\*" ...
bindir = os.path.join(appdata, '.minecraft', 'bin')
bin = os.path.join(bindir, '*')
args.append('"{0}"'.format(bin))

# ... -Djava.library.path="%APPDATA%\.minecraft\bin\natives" ...
libdir = os.path.join(bindir, "natives")
lib = '-Djava.library.path="{0}"'.format(libdir)
args.append(lib)

# ... net.minecraft.client.Minecraft
klass = "net.minecraft.client.Minecraft"
args.append(klass)

env = {"APPDATA": appdata}
sys.stdout.flush()
Popen(args, env=env)