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Does Python have a ternary conditional operator?

If not, is it possible to simulate one concisely using other language constructs?

Python - append vs. extend

What's the difference between the list methods append() and extend()?

Sort a Python dictionary by value

I have a dictionary of values read from two fields in a database: a string field and a numeric field. The string field is unique, so that is the key of the dictionary.

I can sort on the keys, but how can I sort based on the values?

Note: I have read Stack Overflow question How do I sort a list of dictionaries by values of the dictionary in Python? and probably could change my code to have a list of dictionaries, but since I do not really need a list of dictionaries I wanted to know if there is a simpler solution.

What is a metaclass in Python?

What are metaclasses? What do you use them for?

Check whether a file exists using Python

How do I check whether a file exists, using Python, without using a try statement?

Calling an external command in Python

How can I call an external command (as if I'd typed it at the Unix shell or Windows command prompt) from within a Python script?

How can I make a chain of function decorators in Python?

How can I make two decorators in Python that would do the following?

@makebold
@makeitalic
def say():
   return "Hello"

which should return

<b><i>Hello</i></b>

I'm not trying to make HTML this way in a real application, just trying to understand how decorators and decorator chaining works.

How can I merge two Python dictionaries in a single expression?

I have two Python dictionaries, and I want to write a single expression that returns these two dictionaries, merged. The update() method would be what I need, if it returned its result instead of modifying a dict in-place.

>>> x = {'a':1, 'b': 2}
>>> y = {'b':10, 'c': 11}
>>> z = x.update(y)
>>> print z
None
>>> x
{'a': 1, 'b': 10, 'c': 11}

How can I get that final merged dict in z, not x?

(To be extra-clear, the last-one-wins conflict-handling of dict.update() is what I'm looking for as well.)

What does the yield keyword do in Python?

What is the use of the yield keyword in Python? What does it do?

For example, I'm trying to understand this code¹:

def node._get_child_candidates(self, distance, min_dist, max_dist):
    if self._leftchild and distance - max_dist < self._median:
        yield self._leftchild
    if self._rightchild and distance + max_dist >= self._median:
        yield self._rightchild  

And this is the caller:

result, candidates = list(), [self]
while candidates:
    node = candidates.pop()
    distance = node._get_dist(obj)
    if distance <= max_dist and distance >= min_dist:
        result.extend(node._values)
    candidates.extend(node._get_child_candidates(distance, min_dist, max_dist))
return result

What happens when the method _get_child_candidates is called? A list is returned? A single element is returned? Is it called again? When will subsequent calls stop?

_{1. The code comes from Jochen Schulz (jrschulz), who made a great Python library for metric spaces. This is the link to the complete source: Module mspace.}

In Python, check if a directory exists and create it if necessary

What is the most elegant way to check if the directory a file is going to be written to exists, and if not, create the directory using Python? Here is what I tried:

filename = "/my/directory/filename.txt"
dir = os.path.dirname(filename)

try:
    os.stat(dir)
except:
    os.mkdir(dir)       

f = file(filename)

Somehow, I missed os.path.exists (thanks kanja, Blair, and Douglas). This is what I have now:

def ensure_dir(f):
    d = os.path.dirname(f)
    if not os.path.exists(d):
        os.makedirs(d)

Is there a flag for "open", that makes this happen automatically?

What is the difference between @staticmethod and @classmethod in Python?

What is the difference between a function decorated with @staticmethod and one decorated with @classmethod?

Best way to check if a list is empty

For example, if passed the following:

a = []

How do I check to see if a is empty?

Is there a way to run Python on Android?

We are working on an S60 version and this platform has a nice Python API.

However, there is nothing official about Python on Android, but since Jython exists, is there a way to let the snake and the robot work together?

How do I pass a variable by reference?

The Python documentation seems unclear about whether parameters are passed by reference or value, and the following code produces the unchanged value 'Original'

class PassByReference:
    def __init__(self):
        self.variable = 'Original'
        self.Change(self.variable)
        print self.variable

    def Change(self, var):
        var = 'Changed'

Is there something I can do to pass the variable by actual reference?

What IDE to use for Python?

What IDEs ("GUIs/editors") do others use for Python coding?