python：在exec语句中获取打印输出

Question

我想获取一个exec(...)的输出。以下是我的代码：

code = """
i = [0,1,2]
for j in i :
    print j
"""
result = exec(code)

我该如何获取那些打印出来的内容呢？我想得到类似这样的结果：

0
1
2

谢谢你的帮助。

Answer 1

这是一个适合Python 3的版本，参考了@Jochen的回答。我还添加了try-except语句，这样如果代码出错的话，可以进行错误处理。

import sys
from io import StringIO
import contextlib

@contextlib.contextmanager
def stdoutIO(stdout=None):
    old = sys.stdout
    if stdout is None:
        stdout = StringIO()
    sys.stdout = stdout
    yield stdout
    sys.stdout = old

code = """
i = [0,1,2]
for j in i :
    print(j)
"""
with stdoutIO() as s:
    try:
        exec(code)
    except:
        print("Something wrong with the code")
print("out:", s.getvalue())

Answer 2

你可以在执行某个操作时，把标准输出重定向到一个字符串中，这样你就能捕捉到输出的内容。

Python2

import sys
from cStringIO import StringIO

code = """
i = [0,1,2]
for j in i:
    print(j)
"""

old_stdout = sys.stdout
redirected_output = sys.stdout = StringIO()
exec(code)
sys.stdout = old_stdout

print(redirected_output.getvalue())

Python3

import sys
from io import StringIO

code = """
i = [0,1,2]
for j in i:
    print(j)
"""

old_stdout = sys.stdout
redirected_output = sys.stdout = StringIO()
exec(code)
sys.stdout = old_stdout

print(redirected_output.getvalue())

Answer 3

从Python 3.4开始，标准库里有一个解决方案：https://docs.python.org/3/library/contextlib.html#contextlib.redirect_stdout

from io import StringIO
from contextlib import redirect_stdout

f = StringIO()
with redirect_stdout(f):
    help(pow)
s = f.getvalue()

在旧版本中，你可以写一个上下文管理器来处理替换标准输出（stdout）：

import sys
from io import StringIO
import contextlib

@contextlib.contextmanager
def stdoutIO(stdout=None):
    old = sys.stdout
    if stdout is None:
        stdout = StringIO()
    sys.stdout = stdout
    yield stdout
    sys.stdout = old

code = """
i = [0,1,2]
for j in i :
    print j
"""
with stdoutIO() as s:
    exec(code)

print("out:", s.getvalue())