Python subprocess does not transfer arguments

I want to automate the process of extracting data from large OSM files with osmosis, but I am having issues running this code snippet to automate tile creation from OSM data:

import sys
import subprocess

def create_tile_pbf(pbf_file, tile_lng, tile_lat):
    """Runs the osmosis tile creator"""
    app_path = "osmosis/bin/"
    app = "osmosis.bat"
    proc = subprocess.Popen([
        app_path + app,
        "--read-pbf", pbf_file,
        "--bounding-box",
        "top=%d" % (tile_lat+1),
        "left=%d" % (tile_lng),
        "bottom=%d" % (tile_lat),
        "right=%d" % (tile_lng+1),
        "--write-pbf", "someotherfile.osm.pbf"
        ], cwd=app_path, shell=True)
    # Poll the proc to see if it is finished
    while proc.returncode is None:
        proc.communicate()
        proc.poll()
    print(">> Terminated\n")

if __name__ == "__main__":
    print("Args were: " + str(sys.argv) + "\n")
    create_tile_pbf("./somefile.osm.pbf", 7, 46)

I tried with and without shell=True, I tried joining all arguments into a single string. But I always get this error when executing it:

Nov 03, 2017 2:26:28 PM org.openstreetmap.osmosis.core.Osmosis main
SEVERE: Execution aborted.
org.openstreetmap.osmosis.core.OsmosisRuntimeException: Expected argument 1 to be an option or task name.
        at org.openstreetmap.osmosis.core.cli.CommandLineParser.parse(CommandLineParser.java:79)
        at org.openstreetmap.osmosis.core.Osmosis.run(Osmosis.java:74)
        at org.openstreetmap.osmosis.core.Osmosis.main(Osmosis.java:37)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
        at java.lang.reflect.Method.invoke(Unknown Source)
        at org.codehaus.plexus.classworlds.launcher.Launcher.launchStandard(Launcher.java:330)
        at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:238)
        at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:415)
        at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:356)
        at org.codehaus.classworlds.Launcher.main(Launcher.java:47)

But when running the command with Powershell or command prompt, it works like a charm.

.\osmosis.bat --read-pbf .\somefile.osm.pbf --bounding-box top=47 left=7 bottom=46 right=8 --write-pbf someotherfile.osm.pbf

I am running Windows 10 1703 64-bit with Anaconda 4.3.30.

The difference between your script and your command prompt interaction is osmosis/bin/osmosis.bat vs. .\osmosis.bat. Since your call to Popen() already includes the cwd option, you don't need to specify the directory again. That means your call should be:

proc = subprocess.Popen([
    app,                                    # <== No app_path here
    "--read-pbf", pbf_file,
    "--bounding-box",
    "top=%d" % (tile_lat+1),
    "left=%d" % (tile_lng),
    "bottom=%d" % (tile_lat),
    "right=%d" % (tile_lng+1),
    "--write-pbf", "someotherfile.osm.pbf"
    ], cwd=app_path, shell=True)           # <== because of this cwd