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 Defaults for lambda functions:
167
168 f = lambda x, y=default: ...
169
170 Defines instance of f with method:
171
172 def <lambda>(<context>, x, y=default):
173 ...
174
175 Where default is attribute #1.
176
177 f(obj) # f not known at compile-time
178
179 f -> f
180 -> load context for argument #1 (f, since an instance is referenced)
181 obj -> argument #2
182
183 Functions as methods:
184
185 def f(x, y, z): ...
186 class C:
187 m = f
188 c = C()
189 ...
190 f(obj, 1, 2) # no restrictions on obj
191 obj.m(1, 2) # f(obj, 1, 2)
192 C.m(obj, 1, 2) # f(obj "assert isinstance(obj, C)", 1, 2)
193
194 Context propagation:
195
196 fn = C.m # has context C
197 fn(obj, 1, 2) # non-instance context -> explicit context required
198 # must perform isinstance(obj, C)
199 fn = c.m # table entry for m on C -> replace context
200 # gives context c
201 fn(1, 2) # instance context -> no explicit context required
202 # context c inserted in call
203
204 Star parameters are a convenience:
205
206 max(1, 2, 3) # call to max(*args) where args == (1, 2, 3)
207 max((1, 2, 3)) # but why not just do this instead?
208
209 One motivation: avoid explicitly making sequences.
210 Opportunity: avoid expensive dynamic allocation of sequences?
211
212 Star parameters, approach #1:
213
214 Make a sequence to hold the extra arguments, either in the caller for known
215 callables or in the function itself.
216
217 Such a sequence would need allocating and its contents copying from the
218 stack.
219
220 Star parameters, approach #2:
221
222 Keep the extra arguments in the stack.
223
224 Access to the star parameter would need to consider assignment to other
225 things and "escape situations" for the parameter:
226
227 def f(*args):
228 return args # need to allocate and return the sequence
229
230 Access to elements of the extra argument sequence would behave slightly
231 differently to normal sequences, but this could be identified at
232 compile-time.
233
234 Star parameters, known callables and sequences, approach #1:
235
236 g(1, 2, 3, 4) # g known as function g(a, *args) at compile-time
237
238 g -> don't get any context information
239 1 -> argument #1
240 2 -> reference to sequence containing arguments #2, #3, #4
241
242 Star parameters, known callables and sequences, approach #2:
243
244 g(1, 2, 3, 4) # g known as function g(a, *args) at compile-time
245
246 g -> don't get any context information
247 1 -> argument #1
248 2 -> argument #2
249 3 -> argument #3
250 4 -> argument #4
251
252 Star parameters, unknown callables, both approach #1 and #2:
253
254 g(1, 2, 3, 4) # g not known at compile-time
255
256 g -> g
257 -> load context for argument #1
258 1 -> argument #2
259 2 -> argument #3
260 3 -> argument #4
261 4 -> argument #5
262
263 Then, check the context and shift the frame if necessary (described above).
264
265 If g has a star parameter - g(a, *args) - then...
266
267 Approach #1 - move arguments #3, #4, #5 (or shifted to #2, #3, #4) into a
268 sequence, adding a reference to the sequence in their place
269
270 Approach #2 - maintain special access rules to arguments #3, #4, #5 (perhaps
271 shifted to #2, #3, #4) as a C-like array
272
273 Tradeoffs for star parameter approaches:
274
275 Approach #1 - potentially costly at run-time as arguments need moving around,
276 but the arguments would behave normally in functions
277
278 Approach #2 - need to track usage of the star parameter and to possibly copy
279 its contents if assigned, as well as providing special access
280 mechanisms, but the invocation procedure would be simpler
