pyinfo.py not compatible with Python 2

[itoijala-huld]

Bug Report

https://github.com/python/mypy/blob/master/mypy/pyinfo.py#L22
uses sys.base_prefix, but this does not exist in Python 2.

The code should check whether base_prefix exists, and if not return a copy of prefix instead.

To Reproduce

Use the options --py2 and --python-executable.
Mypy will try to execute pyinfo.py using the given interpreter, which will fail.

Actual Behavior

Traceback (most recent call last):
  File "/opt/conda/lib/python3.9/runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/opt/conda/lib/python3.9/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/opt/conda/lib/python3.9/site-packages/mypy/__main__.py", line 34, in <module>
    console_entry()
  File "/opt/conda/lib/python3.9/site-packages/mypy/__main__.py", line 12, in console_entry
    main(None, sys.stdout, sys.stderr)
  File "mypy/main.py", line 96, in main
    svf.nsvf.init_handler,
  File "mypy/main.py", line 173, in run_build
  File "mypy/build.py", line 180, in build
  File "mypy/build.py", line 212, in _build
  File "mypy/modulefinder.py", line 782, in compute_search_paths
  File "mypy/modulefinder.py", line 624, in get_prefixes
    import getopt
  File "/opt/conda/lib/python3.9/subprocess.py", line 424, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  File "/opt/conda/lib/python3.9/subprocess.py", line 528, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['.../python2', '/opt/conda/lib/python3.9/site-packages/mypy/pyinfo.py', 'getprefixes']' returned non-zero exit status 1.

Running the command manually gives

Traceback (most recent call last):
  File "/opt/conda/lib/python3.9/site-packages/mypy/pyinfo.py", line 43, in <module>
    print(repr(getprefixes()))
  File "/opt/conda/lib/python3.9/site-packages/mypy/pyinfo.py", line 22, in getprefixes
    return sys.base_prefix, sys.prefix
AttributeError: 'module' object has no attribute 'base_prefix'

Your Environment

• Mypy version used: 0.931
• Python version used: 2.7.5, 2.7.17

[hauntsaninja]

Thanks for the report, want to make a PR?

[sobolevn]

This stas says that less that 0.04% of downloads are for python2: https://pypistats.org/packages/mypy

However, I am not sure how accurate this number is.

[JelleZijlstra]

That's not very informative because mypy does not run on Python 2 (and I believe it never has). It still supports type checking Python 2, but to do that you need to download it using 3.6+ first.