On Windows, I am getting a `SyntaxError` when I try to input commands after having launched a Python 2.7.x interpreter with `subprocess.call`. 

This is a minimal example: 

    import os
    import subprocess
    
    
    def python_env_path(python_path):
        env = os.environ.copy()
        python_scripts = os.path.join(python_path, "Scripts")
        python_bin = os.path.join(python_path, "Library", "bin")
        path_env = "%s;%s;%s;" % (python_path, python_scripts, python_bin)
        env['PATH'] = path_env.encode()
        return env
    
    
    def open_python_prompt(python_path):
        env = python_env_path(python_path)
        prc = subprocess.call(["start", "python"],
                              shell=True, cwd=python_path, env=env)
        if prc != 0:
            print("Unable to open a Python prompt")
            return False
    
        return True
    
    open_python_prompt("C:\Py27x64")

When I try to write whatever simple command for the interpreter I get:

    >>> a = 0
      File "<stdin>", line 1
        a = 0
             ^
    SyntaxError: invalid syntax

I did not find SO questions that solve my issue.
The same code works fine with Python 3.x.
The same Python installation works fine if I open a shell and call the interpreter using a batch file.

The error you're getting indicates that stdin is set to binary mode for some reason. You can see this using the -u command line option:

    C:\>py -2 -u
    Python 2.7.10 (default, May 23 2015, 09:44:00) 
    [MSC v.1500 64 bit (AMD64)] on win32
    Type "help", "copyright", "credits" or "license" for more information.
    >>> a = 0
      File "<stdin>", line 1
        a = 0
             ^
    SyntaxError: invalid syntax

By any chance do you have PYTHONUNBUFFERED set in the environment?

No, I don't. I have just double-checked it. 

Did you try to run my minimal example on your machine? If yes, are you getting the same behavior?

Your example worked fine for me with the official 64-bit distribution of 2.7.10 installed in "C:\Program Files\Python27" on Windows 10.  I created a junction from "C:\Py27x64" to the 2.7 installation directory in order to run the example as provided. 

This issue is filed under "Extension Modules". Is this a personal build of Python? What's in "Library\bin" (python_bin)? That's not a standard Windows Python directory.

It worked also for me with the official 64-bit distribution! I get that buggy behavior with the Anaconda distribution (this was the "Library\bin"). 

I should have checked it as first step! My apologies and thank you so much for your kind help.