inspect: Implement a very basic signature function.

This implements a very basic `inspect.signature()` function.

At the moment it returns only a simple `Signature` instance with a
`parameters` attribute that holds an `OrderedDict` whose length matches the
arity of the input function (the number of arguments it takes).

So, the following code works and is compatible with CPython:

    def f(a, b, *, c):
        pass

    print(len(inspect.signature(f).parameters))

That should print 3.

Signed-off-by: Damien George <[email protected]>

Author: dpgeorge

python-stdlib/inspect/inspect.py

@@ -80,3 +80,60 @@ def currentframe():
 
 def getframeinfo(frame, context=1):
     return ("<unknown>", -1, "<unknown>", [""], 0)
+
+
+class Signature:
+    pass
+
+
+# This `signature()` function is very limited.  It's main purpose is to work out
+# the arity of the given function, ie the number of arguments it takes.
+#
+# The return value is an instance of `Signature` with a `parameters` member which
+# is an OrderedDict whose length is the number of arguments of `f`.
+def signature(f):
+    import collections
+    import uctypes
+
+    s = Signature()
+    s.parameters = collections.OrderedDict()
+
+    t = type(f)
+    if t is type(globals):
+        # A zero-parameter built-in.
+        num_args = 0
+    elif t is type(abs):
+        # A one-parameter built-in.
+        num_args = 1
+    elif t is type(hasattr):
+        # A two-parameter built-in.
+        num_args = 2
+    elif t is type(setattr):
+        # A three-parameter built-in.
+        num_args = 3
+    elif t is type(signature):
+        # A bytecode function, work out the number of arguments by inspecting the bytecode data.
+        fun_obj = uctypes.struct(id(f), (uctypes.ARRAY | 0, uctypes.LONG | 4))
+        bytecode = uctypes.bytearray_at(fun_obj[3], 8)
+        # See py/bc.h:MP_BC_PRELUDE_SIG_DECODE_INTO macro.
+        i = 0
+        z = bytecode[i]
+        i += 1
+        A = z & 0x3
+        K = 0
+        n = 0
+        while z & 0x80:
+            z = bytecode[i]
+            i += 1
+            A |= (z & 0x4) << n
+            K |= ((z & 0x08) >> 3) << n
+        num_args = A + K
+    else:
+        raise NotImplementedError("unsupported function type")
+
+    # Add dummy arguments to the OrderedDict.
+    for i in range(num_args):
+        a = "x{}".format(i)
+        s.parameters[a] = a
+
+    return s

python-stdlib/inspect/manifest.py

@@ -1,3 +1,3 @@
-metadata(version="0.1.3")
+metadata(version="0.2.0")
 
 module("inspect.py")

python-stdlib/inspect/test_inspect.py

@@ -1,3 +1,4 @@
+import collections
 import inspect
 import unittest
 
@@ -58,3 +59,15 @@ def test_isclass(self):
 
     def test_ismodule(self):
         self._test_is_helper(inspect.ismodule, entities[6])
+
+    def test_signature(self):
+        self.assertEqual(inspect.signature(globals).parameters, collections.OrderedDict())
+        self.assertEqual(len(inspect.signature(abs).parameters), 1)
+        self.assertEqual(len(inspect.signature(hasattr).parameters), 2)
+        self.assertEqual(len(inspect.signature(setattr).parameters), 3)
+        self.assertEqual(len(inspect.signature(lambda: 0).parameters), 0)
+        self.assertEqual(len(inspect.signature(lambda x: 0).parameters), 1)
+        self.assertEqual(len(inspect.signature(lambda *, x: 0).parameters), 1)
+        self.assertEqual(len(inspect.signature(lambda x, y: 0).parameters), 2)
+        self.assertEqual(len(inspect.signature(lambda x, y, z: 0).parameters), 3)
+        self.assertEqual(len(inspect.signature(lambda x, y, *, z: 0).parameters), 3)