Might be clearer to do something like:
```python
def coffee_cmd(watch=False):
    tpl = "{precmd} {coffee} {flags} {dir}"
    flags = ["--compile"]
    if watch:
        flags.append("--watch")
    if platform.system() == "Darwin":
        precmd = "ulimit -n 8000;"
    else:
        precmd = ""
    return tpl.format(
        precmd=precmd,
        coffee="node_modules/.bin/coffee",
        flags=" ".join(flags),
        dir="."
    )
```