如何避免两次写request.GET.get()以便打印？

Question

我之前是做PHP的，想知道在Python里有没有类似的做法。

在PHP中，你可以一举两得，像这样：

而不是：

if(getData()){
    $data = getData();
    echo $data;
}

我可以这样做：

if($data = getData()){
    echo $data;
}

你可以检查一下 getData() 这个函数是否存在，如果存在的话，就可以在一个语句里把它赋值给一个变量。

我想知道在Python中有没有类似的方式？也就是说，不用重复写：

if request.GET.get('q'):
    q = request.GET.get('q')
    print q

避免两次写 request.GET.get('q')。

Answer 1

这是对Alex回答的一个变体：

class DataHolder:
    def __init__(self, value=None, attr_name='value'):
        self._attr_name = attr_name
        self.set(value)
    def __call__(self, value):
        return self.set(value)
    def set(self, value):
        setattr(self, self._attr_name, value)
        return value
    def get(self):
        return getattr(self, self._attr_name)
save_data = DataHolder()

用法：

if save_data(get_input()):
    print save_data.value

或者如果你喜欢另一种界面：

if save_data.set(get_input()):
    print save_data.get()

我觉得这个方法在测试一系列正则表达式时很有用，特别是在使用if-elif-elif-elif等结构时，就像在这个SO问题中提到的那样：

import re

input = u'test bar 123'
save_match = DataHolder(attr_name='match')
if save_match(re.search('foo (\d+)', input)):
    print "Foo"
    print save_match.match.group(1)
elif save_match(re.search('bar (\d+)', input)):
    print "Bar"
    print save_match.match.group(1)
elif save_match(re.search('baz (\d+)', input)):
    print "Baz"
    print save_match.match.group(1)

Answer 2

你可以在这里查看我8年前写的这个任务的做法：这里。

# In Python, you can't code "if x=foo():" -- assignment is a statement, thus
# you can't fit it into an expression, as needed for conditions of if and
# while statements, &c.  No problem, if you just structure your code around
# this.  But sometimes you're transliterating C, or Perl, or ..., and you'd
# like your transliteration to be structurally close to the original.
#
# No problem, again!  One tiny, simple utility class makes it easy...:

class DataHolder:
    def __init__(self, value=None): self.value = value
    def set(self, value): self.value = value; return value
    def get(self): return self.value
# optional but handy, if you use this a lot, either or both of:
setattr(__builtins__,'DataHolder',DataHolder)
setattr(__builtins__,'data',DataHolder())

# and now, assign-and-set to your heart's content: rather than Pythonic
while 1:
    line = file.readline()
    if not line: break
    process(line)
# or better in modern Python, but quite far from C-like idioms:
for line in file.xreadlines():
    process(line)
# you CAN have your C-like code-structure intact in transliteration:
while data.set(file.readline()):
    process(data.get())

Answer 3

可能不是你想的那样，但是...

q = request.GET.get('q')
if q:
    print q

这样吗？