Non-intrusive API for calling mypy directly from another python application. (#2439)

(See also #2114 which was a more complex version.)

Author: JdeH

mypy/api.py

@@ -0,0 +1,56 @@
+"""This module makes it possible to use mypy as part of a Python application.
+
+Since mypy still changes, the API was kept utterly simple and non-intrusive.
+It just mimics command line activation without starting a new interpreter.
+So the normal docs about the mypy command line apply.
+Changes in the command line version of mypy will be immediately useable.
+
+Just import this module and then call the 'run' function with exactly the
+string you would have passed to mypy from the command line.
+Function 'run' returns a tuple of strings: (<normal_report>, <error_report>),
+in which <normal_report> is what mypy normally writes to sys.stdout and
+<error_report> is what mypy normally writes to sys.stderr.
+Any pretty formatting is left to the caller.
+
+Trivial example of code using this module:
+
+import sys
+from mypy import api
+
+result = api.run(' '.join(sys.argv[1:]))
+
+if result[0]:
+    print('\nType checking report:\n')
+    print(result[0])  # stdout
+
+if result[1]:
+    print('\nError report:\n')
+    print(result[1])  # stderr
+"""
+
+import sys
+from io import StringIO
+from typing import Tuple
+from mypy.main import main
+
+
+def run(params: str) -> Tuple[str, str]:
+    sys.argv = [''] + params.split()
+
+    old_stdout = sys.stdout
+    new_stdout = StringIO()
+    sys.stdout = new_stdout
+
+    old_stderr = sys.stderr
+    new_stderr = StringIO()
+    sys.stderr = new_stderr
+
+    try:
+        main(None)
+    except:
+        pass
+
+    sys.stdout = old_stdout
+    sys.stderr = old_stderr
+
+    return new_stdout.getvalue(), new_stderr.getvalue()