1 Concepts
2 ========
3
4 This document describes the underlying concepts employed in micropython.
5
6 * Namespaces and attribute definition
7 * Contexts and values
8 * Tables, attributes and lookups
9 * Objects and structures
10 * Parameters and lookups
11 * Instantiation
12
13 Namespaces and Attribute Definition
14 ===================================
15
16 Namespaces are any objects which can retain attributes.
17
18 * Module attributes are defined either at the module level or by global
19 statements.
20 * Class attributes are defined only within class statements.
21 * Instance attributes are defined only by assignments to attributes of self
22 within __init__ methods.
23
24 These restrictions apply because such attributes are thus explicitly declared,
25 permitting the use of tables (described below). Module and class attributes
26 can also be finalised in this way in order to permit certain optimisations.
27
28 See rejected.txt for complicating mechanisms which could be applied to
29 mitigate the effects of these restrictions on optimisations.
30
31 Contexts and Values
32 ===================
33
34 Values are used as the common reference representation in micropython: as
35 stored representations of attributes (of classes, instances, modules, and
36 other objects supporting attribute-like entities) as well as the stored values
37 associated with names in functions and methods.
38
39 Unlike other implementations, micropython does not create things like bound
40 method objects for individual instances. Instead, all objects are referenced
41 using a context, reference pair:
42
43 Value Layout
44 ------------
45
46 0 1
47 context object
48 reference reference
49
50 Specific implementations might reverse this ordering for optimisation
51 purposes.
52
53 Rationale
54 ---------
55
56 To reduce the number of created objects whilst retaining the ability to
57 support bound method invocations. The context indicates the context in which
58 an invocation is performed, typically the owner of the method.
59
60 Usage
61 -----
62
63 The context may be inserted as the first argument when a value is involved in
64 an invocation. This argument may then be omitted from the invocation if its
65 usage is not appropriate.
66
67 See invocation.txt for details.
68
69 Contexts in Acquired Values
70 ---------------------------
71
72 There are two classes of instructions which provide values:
73
74 Instruction Purpose Context Operations
75 ----------- ------- ------------------
76
77 LoadConst Load class, function, Combine null context with
78 module, constant loaded object
79
80 LoadAddress* Load attribute from Preserve or override stored
81 LoadAttr* class, module, context (as described in
82 instance assignment.txt)
83
84 In order to comply with traditional Python behaviour, contexts may or may not
85 represent the object from which an attribute has been acquired.
86
87 See assignment.txt for details.
88
89 Contexts in Stored Values
90 -------------------------
91
92 There is only one class of instruction for storing values:
93
94 Instruction Purpose Context Operations
95 ----------- ------- ------------------
96
97 StoreAddress Store attribute in a Preserve context; note that no
98 known object test for class attribute
99 assignment should be necessary
100 since this instruction should only
101 be generated for module globals
102
103 StoreAttr Store attribute in an Preserve context; note that no
104 instance test for class attribute
105 assignment should be necessary
106 since this instruction should only
107 be generated for self accesses
108
109 StoreAttrIndex Store attribute in an Preserve context; since the index
110 unknown object lookup could yield a class
111 attribute, a test of the nature of
112 the nature of the structure is
113 necessary in order to prevent
114 assignments to classes
115
116 Note that contexts are never changed in the stored value: they are preserved.
117
118 See assignment.txt for details.
119
120 Tables, Attributes and Lookups
121 ==============================
122
123 Attribute lookups, where the exact location of an object attribute is deduced,
124 are performed differently in micropython than in other implementations.
125 Instead of providing attribute dictionaries, in which attributes are found,
126 attributes are located at fixed places in object structures (described below)
127 and their locations are stored using a special representation known as a
128 table.
129
130 For a given program, a table can be considered as being like a matrix mapping
131 classes to attribute names. For example:
132
133 class A:
134 # instances have attributes x, y
135
136 class B(A):
137 # introduces attribute z for instances
138
139 class C:
140 # instances have attributes a, b, z
141
142 This would provide the following table, referred to as an object table in the
143 context of classes and instances:
144
145 Class/attr a b x y z
146
147 A 1 2
148 B 1 2 3
149 C 1 2 3
150
151 A limitation of this representation is that instance attributes may not shadow
152 class attributes: if an attribute with a given name is not defined on an
153 instance, an attribute with the same name cannot be provided by the class of
154 the instance or any superclass of the instance's class.
155
156 The table can be compacted using a representation known as a displacement
157 list (referred to as an object list in this context):
158
159 Classes with attribute offsets
160
161 classcode A
162 attrcode a b x y z
163
164 B
165 a b x y z
166
167 C
168 a b x y z
169
170 List . . 1 2 1 2 3 1 2 . . 3
171
172 Here, the classcode refers to the offset in the list at which a class's
173 attributes are defined, whereas the attrcode defines the offset within a
174 region of attributes corresponding to a single attribute of a given name.
175
176 Attribute Locations
177 -------------------
178
179 The locations stored in table/list elements are for instance attributes
180 relative to the location of the instance, whereas those for class attributes
181 and modules are absolute addresses (although these could also be changed to
182 object-relative locations).
183
184 Objects and Structures
185 ======================
186
187 As well as references, micropython needs to have actual objects to refer to.
188 Since classes, functions and instances are all objects, it is desirable that
189 certain common features and operations are supported in the same way for all
190 of these things. To permit this, a common data structure format is used.
191
192 Header.................................................... Attributes.................
193
194 Identifier Identifier Address Identifier Size Object Object ...
195
196 0 1 2 3 4 5 6 7
197 classcode attrcode/ invocation funccode size __class__ attribute ...
198 instance reference reference reference
199 status
200
201 Classcode
202 ---------
203
204 Used in attribute lookup.
205
206 Here, the classcode refers to the attribute lookup table for the object (as
207 described above). Classes and instances share the same classcode, and their
208 structures reflect this. Functions all belong to the same type and thus employ
209 the classcode for the function built-in type, whereas modules have distinct
210 types since they must support different sets of attributes.
211
212 Attrcode
213 --------
214
215 Used to test instances for membership of classes (or descendants of classes).
216
217 Since, in traditional Python, classes are only ever instances of the "type"
218 built-in class, support for testing such a relationship directly has been
219 removed and the attrcode is not specified for classes: the presence of an
220 attrcode indicates that a given object is an instance.
221
222 See the "Testing Instance Compatibility with Classes (Attrcode)" section below
223 for details of attrcodes.
224
225 Invocation Reference
226 --------------------
227
228 Used when an object is called.
229
230 This is the address of the code to be executed when an invocation is performed
231 on the object.
232
233 Funccode
234 --------
235
236 Used to look up argument positions by name.
237
238 The strategy with keyword arguments in micropython is to attempt to position
239 such arguments in the invocation frame as it is being constructed.
240
241 See the "Parameters and Lookups" section for more information.
242
243 Size
244 ----
245
246 Used to indicate the size of an object including attributes.
247
248 Attributes
249 ----------
250
251 For classes, modules and instances, the attributes in the structure correspond
252 to the attributes of each kind of object. For functions, however, the
253 attributes in the structure correspond to the default arguments for each
254 function, if any.
255
256 Structure Types
257 ---------------
258
259 Class C:
260
261 0 1 2 3 4 5 6 7
262 classcode (unused) __new__ funccode size class type attribute ...
263 for C reference for reference reference
264 instantiator
265
266 Instance of C:
267
268 0 1 2 3 4 5 6 7
269 classcode attrcode C.__call__ funccode size class C attribute ...
270 for C for C reference for reference reference
271 (if exists) C.__call__
272
273 Function f:
274
275 0 1 2 3 4 5 6 7
276 classcode attrcode code funccode size class attribute ...
277 for for reference function (default)
278 function function reference reference
279
280 Module m:
281
282 0 1 2 3 4 5 6 7
283 classcode attrcode (unused) (unused) (unused) module type attribute ...
284 for m for m reference (global)
285 reference
286
287 The __class__ Attribute
288 -----------------------
289
290 All objects support the __class__ attribute and this is illustrated above with
291 the first attribute.
292
293 Class: refers to the type class (type.__class__ also refers to the type class)
294 Function: refers to the function class
295 Instance: refers to the class instantiated to make the object
296
297 Lists and Tuples
298 ----------------
299
300 The built-in list and tuple sequences employ variable length structures using
301 the attribute locations to store their elements, where each element is a
302 reference to a separately stored object.
303
304 Testing Instance Compatibility with Classes (Attrcode)
305 ------------------------------------------------------
306
307 Although it would be possible to have a data structure mapping classes to
308 compatible classes, such as a matrix indicating the subclasses (or
309 superclasses) of each class, the need to retain the key to such a data
310 structure for each class might introduce a noticeable overhead.
311
312 Instead of having a separate structure, descendant classes of each class are
313 inserted as special attributes into the object table. This requires an extra
314 key to be retained, since each class must provide its own attribute code such
315 that upon an instance/class compatibility test, the code may be obtained and
316 used in the object table.
317
318 Invocation and Code References
319 ------------------------------
320
321 Modules: there is no meaningful invocation reference since modules cannot be
322 explicitly called.
323
324 Functions: a simple code reference is employed pointing to code implementing
325 the function. Note that the function locals are completely distinct from this
326 structure and are not comparable to attributes. Instead, attributes are
327 reserved for default parameter values, although they do not appear in the
328 object table described above, appearing instead in a separate parameter table
329 described below.
330
331 Classes: given that classes must be invoked in order to create instances, a
332 reference must be provided in class structures. However, this reference does
333 not point directly at the __init__ method of the class. Instead, the
334 referenced code belongs to a special initialiser function, __new__, consisting
335 of the following instructions:
336
337 create instance for C
338 call C.__init__(instance, ...)
339 return instance
340
341 Instances: each instance employs a reference to any __call__ method defined in
342 the class hierarchy for the instance, thus maintaining its callable nature.
343
344 Both classes and modules may contain code in their definitions - the former in
345 the "body" of the class, potentially defining attributes, and the latter as
346 the "top-level" code in the module, potentially defining attributes/globals -
347 but this code is not associated with any invocation target. It is thus
348 generated in order of appearance and is not referenced externally.
349
350 Invocation Operation
351 --------------------
352
353 Consequently, regardless of the object an invocation is always done as
354 follows:
355
356 get invocation reference from the header
357 jump to reference
358
359 Additional preparation is necessary before the above code: positional
360 arguments must be saved in the invocation frame, and keyword arguments must be
361 resolved and saved to the appropriate position in the invocation frame.
362
363 See invocation.txt for details.
364
365 Parameters and Lookups
366 ======================
367
368 Since Python supports keyword arguments when making invocations, it becomes
369 necessary to record the parameter names associated with each function or
370 method. Just as object tables record attributes positions on classes and
371 instances, parameter tables record parameter positions in function or method
372 parameter lists.
373
374 For a given program, a parameter table can be considered as being like a
375 matrix mapping functions/methods to parameter names. For example:
376
377 def f(x, y, z):
378 pass
379
380 def g(a, b, c):
381 pass
382
383 def h(a, x):
384 pass
385
386 This would provide the following table, referred to as a parameter table in
387 the context of functions and methods:
388
389 Function/param a b c x y z
390
391 f 1 2 3
392 g 1 2 3
393 h 1 2
394
395 Just as with parameter tables, a displacement list can be prepared from a
396 parameter table:
397
398 Functions with parameter (attribute) offsets
399
400 funccode f
401 attrcode a b c x y z
402
403 g
404 a b c x y z
405
406 h
407 a b c x y z
408
409 List . . . 1 2 3 1 2 3 1 . . 2 . .
410
411 Here, the funccode refers to the offset in the list at which a function's
412 parameters are defined, whereas the attrcode defines the offset within a
413 region of attributes corresponding to a single parameter of a given name.
414
415 Instantiation
416 =============
417
418 When instantiating classes, memory must be reserved for the header of the
419 resulting instance, along with locations for the attributes of the instance.
420 Since the instance header contains data common to all instances of a class, a
421 template header is copied to the start of the newly reserved memory region.
