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224 lines
2.1 KiB
Markdown
224 lines
2.1 KiB
Markdown
# 动态编译
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## 标准编程语言
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对于 **C** 语言，代码一般要先编译，再执行。
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```py
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.c -> .exe
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```
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## 解释器语言
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shell 脚本
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```py
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.sh -> interpreter
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```
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## Byte Code 编译
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**Python, Java** 等语言先将代码编译为 byte code（不是机器码），然后再处理：
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```py
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.py -> .pyc -> interpreter
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```
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## eval 函数
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```py
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eval(statement, glob, local)
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```
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使用 `eval` 函数动态执行代码，返回执行的值：
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In [1]:
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```py
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a = 1
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eval("a+1")
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```
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Out[1]:
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```py
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2
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```
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可以接收明明空间参数：
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In [2]:
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```py
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local = dict(a=2)
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glob = {}
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eval("a+1", glob, local)
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```
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Out[2]:
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```py
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3
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```
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这里 `local` 中的 `a` 先被找到。
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## exec 函数
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```py
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exec(statement, glob, local)
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```
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使用 `exec` 可以添加修改原有的变量。
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In [3]:
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```py
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a = 1
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exec("b = a+1")
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print b
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```
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```py
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2
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```
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In [4]:
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```py
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local = dict(a=2)
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glob = {}
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exec("b = a+1", glob, local)
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print local
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```
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```py
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{'a': 2, 'b': 3}
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```
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执行之后，`b` 在 `local` 命名空间中。
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## 警告
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动态执行的时候要注意，不要执行不信任的用户输入，因为它们拥有 `Python` 的全部权限。
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## compile 函数生成 byte code
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```py
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compile(str, filename, mode)
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```
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In [5]:
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```py
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a = 1
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c = compile("a+2", "", 'eval')
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eval(c)
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```
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Out[5]:
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```py
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3
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```
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In [6]:
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```py
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a = 1
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c = compile("b=a+2", "", 'exec')
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exec(c)
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b
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```
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Out[6]:
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```py
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3
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```
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## abstract syntax trees
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In [7]:
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```py
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import ast
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```
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In [8]:
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```py
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tree = ast.parse("a+2", "", "eval")
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ast.dump(tree)
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```
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Out[8]:
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```py
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"Expression(body=BinOp(left=Name(id='a', ctx=Load()), op=Add(), right=Num(n=2)))"
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```
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改变常数的值：
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In [9]:
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```py
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tree.body.right.n = 3
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ast.dump(tree)
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```
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Out[9]:
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```py
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"Expression(body=BinOp(left=Name(id='a', ctx=Load()), op=Add(), right=Num(n=3)))"
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```
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In [10]:
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```py
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a = 1
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c = compile(tree, '', 'eval')
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eval(c)
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```
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Out[10]:
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```py
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4
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```
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安全的使用方法 `literal_eval` ，只支持基本值的操作：
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In [11]:
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```py
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ast.literal_eval("[10.0, 2, True, 'foo']")
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```
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Out[11]:
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```py
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[10.0, 2, True, 'foo']
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``` |