1 Invocations in classic Python:
2
3 f(1, 2, 3) # positional
4 f(1, 2) # positional with defaults
5 f(1, 2, c=3) # keywords
6 f(1, c=3) # keywords with defaults
7 f(1, 2, 3, 4) # extra positional arguments
8 f(1, 2, 3, d=4) # extra keyword arguments
9 f(1, 2, *args) # positional bundles (possibly with defaults)
10 f(1, 2, **kw) # keyword bundles (possibly with defaults)
11
12 Note that f is never fixed before run-time in Python.
13
14 Comparison to invocations in C:
15
16 f(1, 2, 3) # positional, f known at compile-time
17 f(1, 2, 3) # positional, f is appropriate function pointer
18 # ie. (*f)(A, B, C)
19
20 Least expensive cases (positional plus defaults):
21
22 f(1, 2, 3) # put arguments in frame
23 # if f is not known, add arguments vs. parameters check
24 f(1, 2) # to handle defaults, introduce default "filling" where
25 # not enough arguments are given
26 # if f is not known, this is obviously done at run-time
27
28 More expensive cases (keywords plus defaults):
29
30 f(1, 2, c=3) # prepare frame using parameter details
31 # (provided c is a known parameter)
32 # if f is not known, this is obviously done at run-time
33 f(1, c=3) # as with the previous case, with default "filling" done
34 # where not enough arguments are given
35 # if f is not known, this is obviously done at run-time
36 # but with all defaults copied in before keywords are
37 # assigned (since their positions and thus the positions
38 # of missing parameters cannot be known)
39
40 Awkward cases (extra arguments):
41
42 f(1, 2, 3, 4) # put arguments in frame
43 # if f is not known, add arguments vs. parameters check;
44 # to handle superfluous arguments, make a suitable object
45 # and fill it with all such arguments
46
47 Very awkward cases:
48
49 f(1, 2, 3, d=4) # extra keyword arguments
50 f(1, 2, *args) # positional bundles (possibly with defaults)
51 f(1, 2, **kw) # keyword bundles (possibly with defaults)
52
53 These cases require additional structures to be created, potentially at
54 run-time.
55
56 Methods vs. functions:
57
58 f(obj, 1, 2) # f known as function at compile-time:
59 # f(obj, 1, 2)
60 # f known as C.m at compile-time:
61 # m(obj "assert isinstance(obj, C)", 1, 2)
62 # f not known at compile-time:
63 # f(<context>, obj, 1, 2) for instance-accessed methods
64 # f(obj, 1, 2) for class-accessed methods
65 # f(obj, 1, 2) for functions
66
67 (Could either have universal context usage even for functions, which would
68 ignore them, or attempt to remove contexts when functions are called.)
69
70 Argument lists for functions:
71
72 f(obj, 1, 2) # f known as function at compile-time
73
74 f -> don't get any context information
75 obj -> argument #1
76 1 -> argument #2
77 2 -> argument #3
78
79 Argument lists for methods:
80
81 f(obj, 1, 2) # f known as C.m at compile-time (context is C)
82
83 f -> C.m - don't get any context information
84 obj -> argument #1
85 1 -> argument #2
86 2 -> argument #3
87
88 Argument lists for methods:
89
90 f(obj, 1, 2) # f known as C.m at compile-time (context is an instance)
91
92 f -> C.m
93 -> context is argument #1
94 obj -> argument #2
95 1 -> argument #3
96 2 -> argument #4
97
98 Argument lists for classes:
99
100 f(obj, 1, 2) # f known as C at compile-time
101
102 f -> C.__init__
103 -> new instance is argument #1
104 obj -> argument #2
105 1 -> argument #3
106 2 -> argument #4
107
108 The new instance must be manually provided as the result after the call.
109
110 Argument lists for unknown callables:
111
112 f(obj, 1, 2) # f not known at compile-time
113
114 f -> f
115 -> load context for argument #1
116 obj -> argument #2
117 1 -> argument #3
118 2 -> argument #4
119
120 Then, check the context and shift the frame if necessary:
121
122 <context> is module or class:
123 (<context>, obj, 1, 2) -> (obj, 1, 2)
124
125 <context> is instance: no change
126
127 Argument lists in instantiators:
128
129 f(obj, 1, 2) # f not known at compile-time
130
131 f -> C.__new__ (known and called at run-time)
132
133 Need to call C.__init__(<instance>, obj, 1, 2), preferably with the existing
134 frame:
135
136 <insert instance before received arguments>
137 obj -> argument #1
138 1 -> argument #2
139 2 -> argument #3
140
141 Then jump without switching frames.
142 It should be possible to replace the old, tentative context information in the
143 frame.
144
145 Defaults for unknown callables:
146
147 f(obj) # f not known at compile-time
148
149 f -> f
150 -> load context for argument #1
151 obj -> argument #2
152
153 Then, check the number of arguments and the availability of defaults against
154 the details provided by the callable's structure.
155
156 Checking defaults for unknown callables:
157
158 Approach #1 - pre-fill defaults, add arguments, check frame
159
160 Approach #2 - add arguments, add defaults while checking frame
161
162 Functions as methods:
163
164 def f(x, y, z): ...
165 class C:
166 m = f
167 c = C()
168 ...
169 f(obj, 1, 2) # no restrictions on obj
170 obj.m(1, 2) # f(obj, 1, 2)
171 C.m(obj, 1, 2) # f(obj "assert isinstance(obj, C)", 1, 2)
172
173 Context propagation:
174
175 fn = C.m # has context C
176 fn(obj, 1, 2) # non-instance context -> explicit context required
177 # must perform isinstance(obj, C)
178 fn = c.m # table entry for m on C -> replace context
179 # gives context c
180 fn(1, 2) # instance context -> no explicit context required
181 # context c inserted in call
182
183 Star parameters are a convenience:
184
185 max(1, 2, 3) # call to max(*args) where args == (1, 2, 3)
186 max((1, 2, 3)) # but why not just do this instead?
187
188 One motivation: avoid explicitly making sequences.
189 Opportunity: avoid expensive dynamic allocation of sequences?
190
191 Star parameters, known callables and sequences:
192
193 g(1, 2, 3, 4) # g known as function g(a, *args) at compile-time
194
195 g -> don't get any context information
196 1 -> argument #1
197 2 -> reference to sequence containing arguments #2, #3, #4
198
199 (This according to approach #1 described for unknown callables. With approach
200 #2, normal argument positioning would occur.)
201
202 Star parameters, unknown callables:
203
204 g(1, 2, 3, 4) # g not known at compile-time
205
206 g -> g
207 -> load context for argument #1
208 1 -> argument #2
209 2 -> argument #3
210 3 -> argument #4
211 4 -> argument #5
212
213 Then, check the context and shift the frame if necessary (described above).
214
215 If g has a star parameter - g(a, *args) - then...
216
217 Approach #1 - move arguments #3, #4, #5 (or shifted to #2, #3, #4) into a
218 sequence, adding a reference to the sequence in their place
219
220 Approach #2 - maintain special access rules to arguments #3, #4, #5 (perhaps
221 shifted to #2, #3, #4) as a C-like array
222
223 Tradeoffs for star parameter approaches:
224
225 Approach #1 - potentially costly at run-time as arguments need moving around,
226 but the arguments would behave normally in functions
227
228 Approach #2 - need to track usage of the star parameter and to possibly copy
229 its contents if assigned, as well as providing special access
230 mechanisms, but the invocation procedure would be simpler
