How do you pass an io-file-like StringIO object to a subprocess that expects a specific file as input

Question

I am trying to pass a file-like StringIO object to a subprocess which expects a file "file.ly". Possible problems might be the file extension and expected apostrophes before and after the filename.

I tried:

import subprocess
import lilypond
from io import StringIO  

string ='{e c}'
file = StringIO(string)
print(file.read())
file.seek(0)

subprocess.run([lilypond.executable(),"-fpng", file.read()])

The code works as expected when using:

subprocess.run([lilypond.executable(),"-fpng", "file.ly"])

("file.ly" contains "{e c}")

Here is the error message:

{e c} Warnung: Datei »{e c}« kann nicht gefunden werden schwerer Fehler: gescheiterte Dateien: "{e c}"

This translates to the following (using google translate):

{e c} Warning: Cannot find file »{e c}« fatal error: failed files: "{e c}"

I suppose that this means that the python code is OK, but the lilypond subprocess doesn't like the input.

Is there a fix for this problem?