Passing a binary file from Python3 to Python2 on Windows

Question

We have two python scripts, which need to communicate over stdin/out (on Windows). Sadly they both have to be in different Python versions. The snippets are:

Source (Python 3):

sys.stderr.write("LEN1: %s\n" % len(source_file.read()))
subprocess.check_call(["C:\\python2.exe", "-u", "-c",
"""
import foo
foo.to_json()
"""], stdin=source_file, stdout=json_file, stderr=sys.stderr, env=os.environ)

Target (Python 2):

def to_json():
  import msvcrt
  msvcrt.setmode(sys.stdin.fileno(), os.O_BINARY)
  input_message = sys.stdin.read()
  sys.stderr.write("LEN2: %s\n" % len(input_message))

When I execute the scripts, I get:

LEN2: 0
LEN1: 37165

It seems like I am doing something fundamentally wrong, but cannot really figure out, what exactly. Could anyone try to help me debug, where I am going wrong.

Have you tried using a Popen with pipes and talking to it with communicate? docs.python.org/2/library/subprocess.html#subprocess.Popen — theodox
– theodox, Commented May 26, 2015 at 16:47
The setmode call in to_json is redundant since the -u argument sets stdin to binary mode. Passing env=os.environ is probably redundant, unless os.environ has somehow gotten out of sync with the process environment. — Eryk Sun
– Eryk Sun, Commented May 26, 2015 at 17:10

Damian Yerrick · Accepted Answer · 2015-05-26 16:52:49Z

3

After the first source_file.read() to determine the file's length, the file pointer is at the end of the file. By the time you've passed the stream to check_call(), you've exhausted it, and further reads will produce the empty string. This can be worked around in two ways:

A. Read the file into memory before calculating its length.

B. Before check_call(), rewind the file object to the beginning of the file with source_file.seek(0).

answered May 26, 2015 at 16:52

Damian Yerrick

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1 Comment

abergmeier Over a year ago

Thanks. Then seems like all data is received fine.

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