Python3：将递归 function 注入到 function 中的 exec() 中

Question

In python3, if a function with recursive invoking is injected into exec() in a function, I got an error. For example, below code

def B(pys):
    exec(pys)

pys="""
def fibonacci(n):
    if n == 1 or n == 2:
        r = 1
    else:
        r = fibonacci(n - 1) + fibonacci(n - 2)
    return r

print(fibonacci(3))
"""
B(pys)

will raise NameError.

$ py -3.8 testrecursivefun.py
Traceback (most recent call last):
  File "testrecursivefun.py", line 14, in <module>
    B(pys)
  File "testrecursivefun.py", line 2, in B
    exec(pys)
  File "<string>", line 9, in <module>
  File "<string>", line 6, in fibonacci
NameError: name 'fibonacci' is not defined

If I run exec(pys) directly under the module, the exception disappeared.

The reason has been described in another question How does exec work with locals? . But I still don't know how I can figure out the recursive invoking in exec() . Because the function name is dynamic for me. I cannot add it to locals() to exec() . Who can help me figure it out.

Answer 1

For the sake of an answer, you can wrap your code in a function so the recursive function is in its local scope:

import textwrap

def B(pys):
    exec(pys, globals(), {})

pys="""
def fibonacci(n):
    if n == 1 or n == 2:
        r = 1
    else:
        r = fibonacci(n - 1) + fibonacci(n - 2)
    return r

print(fibonacci(11))
"""

def wrap(s):
    return "def foo():\n" \
           "{}\n" \
           "foo()".format(textwrap.indent(s, ' ' * 4))

B(wrap(pys))

Generally, reconsider using exec.

Answer 2

I actually got interested in your question, so I started researching on this topic. Seems like the simple solution to your problem is to:

1. First compile the string to code using compile function in python
2. Then execute the compiled code using exec function

Here is the sample solution:

psy="""
def fibonacci(n):
    if n == 1 or n == 2:
        r = 1
    else:
        r = fibonacci(n - 1) + fibonacci(n - 2)
    return r

print(fibonacci(3))
"""

def B(psy):
    code = compile(psy, '<string>', 'exec')
    exec(code, globals())

B(psy)

Here compile takes three parameters:

First is the code in string format , Second is the filename hint which we usedas we take string as code itself, and third can be one of 'exec', 'eval' and 'single' .

This link contains detail explanation of how you should use exec and eval in python. Do check them out for detail explanation.