The document discusses how Python code is executed. It explains that Python code is first compiled into code objects then executed via a frame object on the stack. Code objects contain details like function names and constants while frame objects contain details like the current code, local and global variables. It also describes the main components involved in execution like opcodes, bytecodes, and the CPython interpreter which evaluates bytecodes by calling corresponding operations.

1. Python 如何執行

2. 演講經歷
● 2013/04 在 taipei.py 演講關於 pdb 的實作。
相關投影片:http://www.slideshare.
net/ya790026/recoverpdb
● 2013/05 在 pyconf.tw 演將 CPython 原始碼
解析。相關投影片:http://www.slideshare.
net/ya790026/c-python-23247730。

3. Python 如何被執行
1. 將程式碼編譯成 code object
2. 執行 code object 的 byte code，變數等相關
資訊放在 frame object 裡。
3. 一個 frame 對應到一個 code object

4. code object
● co_name gives the function name;
● co_varnames is a tuple containing the
names of the local variables (starting with
the argument names);
● co_consts is a tuple containing the literals
used by the bytecode;

5. frame object
● f_back is to the previous stack frame
(towards the caller), or None if this is the
bottom stack frame;
● f_code is the code object being executed in
this frame;
● f_locals is the dictionary used to look up
local variables;
● f_globals is used for global variables;

6. Stack
TOS3 TOS4 TOS1 TOS
BUTTON
TOP

7. op code
● opcode 定義在 Include/opcode.h
● opcode 的長度為一個 byte
● opcode 依照參數種類可分為三種
○ 無參數，如 NOP
○ 參數來源來自 stack，如BINARY_ADD
○ 參數來源來自 opcdoe 後兩個 bype，如
LOAD_CONST
● 如果一個函數使用超過 2^16 個常數?
○ 當 opcdoe 的參數來源來自opcdoe 後兩個 bype，而
該參數無法以 2 byte 來表示。則在該 opcdoe 前插入
EXTENDED_ARG。該 opcode 的參數為
(EXTENDED_ARG 的參數 <<16 | 該opcdoe 的參數)

8. Python Bytecode Instructions
●
●
●
NOP()
Do nothing code. Used as a placeholder
by the bytecode optimizer.
POP_TOP()
Removes the top-of-stack (TOS) item.
ROT_TWO()
Swaps the two top-most stack items.

9. Python Bytecode Instructions
Unary Operations take the top of the stack,
apply the operation, and push the result back
on the stack
●
●
●
UNARY_POSITIVE()
Implements TOS = +TOS.
UNARY_NEGATIVE()
Implements TOS = -TOS.
UNARY_NOT()
Implements TOS = not TOS.

10. Python Bytecode Instructions
Binary operations remove the top of the stack
(TOS) and the second top-most stack item
(TOS1) from the stack. They perform the
operation, and put the result back on the stack.
BINARY_POWER()
Implements TOS = TOS1 ** TOS.
BINARY_MULTIPLY()
Implements TOS = TOS1 * TOS.

11. Python Bytecode Instructions
Slice assignment needs even an additional
parameter. As any statement, they put nothing
on the stack.
STORE_SLICE+0()
Implements TOS[:] = TOS1.
STORE_SLICE+1()
Implements TOS1[TOS:] = TOS2.

12. Miscellaneous opcodes.
LOAD_FAST(var_num)
Pushes a reference to the local
co_varnames[var_num] onto the stack.
LOAD_CONST(consti)
Pushes co_consts[consti] onto the stack.
BUILD_LIST(count)
Works as BUILD_TUPLE, but creates a list.

13. def test(data, data1):
a=5
print 'hello',
print data
return 1
import dis
print dis.dis(test)

14. co_consts: (None, 5, 'hello', 1)
co_varnames: ('data', 'data1', 'a')

15. 2
3
4
5
0 LOAD_CONST
3 STORE_FAST
6 LOAD_CONST
9 PRINT_ITEM
10 LOAD_FAST
13 PRINT_ITEM
14 PRINT_NEWLINE
15 LOAD_CONST
18 RETURN_VALUE
1 (5)
2 (a)
2 ('hello')
0 (data)
3 (1)

16. def test():
print 'hello',
print 'hello',
import dis
print dis.dis(test)

17. 2
0 LOAD_CONST
3 PRINT_ITEM
1 ('hello')
3
4 LOAD_CONST
7 PRINT_ITEM
8 LOAD_CONST
11 RETURN_VALUE
1 ('hello')
0 (None)

18. def printf1(name, **kwargs):
print kwargs[name]
code = printf1.func_code
import dis
print dis.dis(code)

19. 8
0 LOAD_FAST
3 LOAD_FAST
6 BINARY_SUBSCR
7 PRINT_ITEM
8 PRINT_NEWLINE
9 LOAD_CONST
12 RETURN_VALUE
1 (kwargs)
0 (name)
0 (None)

20. def test1():
a=1
b=2
c=a+b
d=a*b
def test2():
a, b = 1, 2
c, d = a + b, a * b

21. 5
0 (a)
6 LOAD_CONST
2 (2)
9 STORE_FAST
1 (b)
12 LOAD_FAST
0 (a)
15 LOAD_FAST
7
1 (1)
3 STORE_FAST
6
0 LOAD_CONST
1 (b)
18 BINARY_ADD
19 STORE_FAST
8
2 (c)
22 LOAD_FAST
0 (a)
25 LOAD_FAST
1 (b)
28 BINARY_MULTIPLY
29 STORE_FAST
3 (d)
32 LOAD_CONST
0 (None)
35 RETURN_VALUE

22. 11
0 LOAD_CONST
3 ((1, 2))
3 UNPACK_SEQUENCE
2
6 STORE_FAST
9 STORE_FAST
12
0 (a)
1 (b)
12 LOAD_FAST
0 (a)
15 LOAD_FAST
1 (b)
18 BINARY_ADD
19 LOAD_FAST
0 (a)
22 LOAD_FAST
1 (b)
25 BINARY_MULTIPLY
26 ROT_TWO
27 STORE_FAST
2 (c)
30 STORE_FAST
3 (d)
33 LOAD_CONST
0 (None)
36 RETURN_VALUE

23. PyEval_EvalFrameEx
● Python/ceval.c
● 負責解讀 opcdoe 並呼叫對應的動作

24. BINARY_ADD
1. 都是整數的話，直接相加
2. 都是字串的話，呼叫 string_concatenate
3. 不滿足上面條件，則呼叫 PyNumber_Add ( in
Object/typeobject.c)。他會去呼叫 __add__
4. 從 stack 取得兩個參數，放回一個結果

25. 結論
1. 比較長的程式碼，op code 未必比較長
2. python 是一個程式碼大量重用的程式

26. Olis
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27. 謝謝大家
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