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 -> f (context is null)
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 (context is class C)
84 obj -> argument #1 (must be tested against the context)
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 -> instantiator of C
103 -> (argument #1 reserved for a new instance made by the instantiator)
104 obj -> argument #2
105 1 -> argument #3
106 2 -> argument #4
107
108 The new instance must be provided as the result of 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 f is class: no change
123
124 <context> is class:
125 (<context>, obj, 1, 2) -> (obj, 1, 2)
126
127 <context> is instance: no change
128
129 Argument lists in instantiators:
130
131 f(obj, 1, 2) # f not known at compile-time
132
133 f -> C.__new__ (known and called at run-time)
134 -> load context for argument #1
135 obj -> argument #2
136 1 -> argument #3
137 2 -> argument #4
138
139 Need to call C.__init__(<instance>, obj, 1, 2), preferably with the existing
140 frame:
141
142 *** -> instance overwrites argument #1
143 obj -> argument #2
144 1 -> argument #3
145 2 -> argument #4
146
147 Then jump without switching frames.
148
149 Defaults for unknown callables:
150
151 f(obj) # f not known at compile-time
152
153 f -> f
154 -> load context for argument #1
155 obj -> argument #2
156
157 Then, check the number of arguments and the availability of defaults against
158 the details provided by the callable's structure.
159
160 Checking defaults for unknown callables:
161
162 Approach #1 - pre-fill defaults, add arguments, check frame
163
164 Approach #2 - add arguments, add defaults while checking frame
165
166 Dynamic functions:
167
168 def f(x):
169 def g(y=x): # dynamic: y depends on non-constant value
170 ...
171 def h(y=2): # static: y depends on constant value
172 ...
173
174 def f(x):
175 g = lambda y=x: ... # dynamic: y depends on non-constant value
176 h = lambda y=2: ... # static: y depends on constant value
177
178 Representation of dynamic functions:
179
180 f = lambda x, y=nonconst: ...
181
182 def f(x, y=nonconst):
183 ...
184
185 Defines instance with method:
186
187 def <lambda>(<context>, x, y=nonconst):
188 ...
189
190 def f(<context>, x, y=nonconst):
191 ...
192
193 Where default is attribute #1.
194
195 f(obj) # f not known at compile-time
196
197 f -> f
198 -> load context for argument #1 (f, since an instance is referenced)
199 obj -> argument #2
200
201 Functions as methods:
202
203 def f(x, y, z): ...
204 class C:
205 m = f
206 c = C()
207 ...
208 f(obj, 1, 2) # no restrictions on obj
209 obj.m(1, 2) # f(obj, 1, 2)
210 C.m(obj, 1, 2) # f(obj "assert isinstance(obj, C)", 1, 2)
211
212 Context propagation:
213
214 fn = C.m # has context C
215 fn(obj, 1, 2) # non-instance context -> explicit context required
216 # must perform isinstance(obj, C)
217 fn = c.m # table entry for m on C -> replace context
218 # gives context c
219 fn(1, 2) # instance context -> no explicit context required
220 # context c inserted in call
221
222 Star parameters are a convenience:
223
224 max(1, 2, 3) # call to max(*args) where args == (1, 2, 3)
225 max((1, 2, 3)) # but why not just do this instead?
226
227 One motivation: avoid explicitly making sequences.
228 Opportunity: avoid expensive dynamic allocation of sequences?
229
230 Star parameters, approach #1:
231
232 Make a sequence to hold the extra arguments, either in the caller for known
233 callables or in the function itself.
234
235 Such a sequence would need allocating and its contents copying from the
236 stack.
237
238 Star parameters, approach #2:
239
240 Keep the extra arguments in the stack.
241
242 Access to the star parameter would need to consider assignment to other
243 things and "escape situations" for the parameter:
244
245 def f(*args):
246 return args # need to allocate and return the sequence
247
248 Access to elements of the extra argument sequence would behave slightly
249 differently to normal sequences, but this could be identified at
250 compile-time.
251
252 Star parameters, known callables and sequences, approach #1:
253
254 g(1, 2, 3, 4) # g known as function g(a, *args) at compile-time
255
256 g -> don't get any context information
257 1 -> argument #1
258 2 -> reference to sequence containing arguments #2, #3, #4
259
260 Star parameters, known callables and sequences, approach #2:
261
262 g(1, 2, 3, 4) # g known as function g(a, *args) at compile-time
263
264 g -> don't get any context information
265 1 -> argument #1
266 2 -> argument #2
267 3 -> argument #3
268 4 -> argument #4
269
270 Star parameters, unknown callables, both approach #1 and #2:
271
272 g(1, 2, 3, 4) # g not known at compile-time
273
274 g -> g
275 -> load context for argument #1
276 1 -> argument #2
277 2 -> argument #3
278 3 -> argument #4
279 4 -> argument #5
280
281 Then, check the context and shift the frame if necessary (described above).
282
283 If g has a star parameter - g(a, *args) - then...
284
285 Approach #1 - move arguments #3, #4, #5 (or shifted to #2, #3, #4) into a
286 sequence, adding a reference to the sequence in their place
287
288 Approach #2 - maintain special access rules to arguments #3, #4, #5 (perhaps
289 shifted to #2, #3, #4) as a C-like array
290
291 Tradeoffs for star parameter approaches:
292
293 Approach #1 - potentially costly at run-time as arguments need moving around,
294 but the arguments would behave normally in functions
295
296 Approach #2 - need to track usage of the star parameter and to possibly copy
297 its contents if assigned, as well as providing special access
298 mechanisms, but the invocation procedure would be simpler