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python interview questions

Top python frequently asked interview questions

How to list all files of a directory in Python

How can I list all files of a directory in python and add them to a list?


Source: (StackOverflow)

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?


Source: (StackOverflow)

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?


Source: (StackOverflow)

How can I represent an 'Enum' in Python?

I'm mainly a C# developer, but I'm currently working on a project in Python.

How can I represent the equivalent of an Enum in Python?


Source: (StackOverflow)

Python - append vs. extend

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


Source: (StackOverflow)

Add key to a dictionary in Python?

Is it possible to add a key to a Python dictionary after it has been created? It doesn't seem to have an .add() method.


Source: (StackOverflow)

Does Python have a ternary conditional operator?

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


Source: (StackOverflow)

What does `if __name__ == "__main__":` do?

What does the if __name__ == "__main__": do?

# Threading example
import time, thread

def myfunction(string, sleeptime, lock, *args):
    while 1:
        lock.acquire()
        time.sleep(sleeptime)
        lock.release()
        time.sleep(sleeptime)
if __name__ == "__main__":
    lock = thread.allocate_lock()
    thread.start_new_thread(myfunction, ("Thread #: 1", 2, lock))
    thread.start_new_thread(myfunction, ("Thread #: 2", 2, lock))

Source: (StackOverflow)

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.


Source: (StackOverflow)

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?


Source: (StackOverflow)

Parse String to Float or Int

In Python, how can I parse a numeric string like "545.2222" to its corresponding float value, 542.2222? Or parse the string "31" to an integer, 31?

I just want to know how to parse a float string to a float, and (separately) an int string to an int.


Source: (StackOverflow)

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.)


Source: (StackOverflow)

Finding the index of an item given a list containing it in Python

For a list ["foo", "bar", "baz"] and an item in the list "bar", what's the cleanest way to get its index (1) in Python?


Source: (StackOverflow)

Does Python have a string contains substring method?

I'm looking for a string.contains or string.indexof method in Python.

I want to do:

if not somestring.contains("blah"):
   continue

Source: (StackOverflow)

Accessing the index in Python for loops

Does anyone know how to access the index itself for a list like this:

ints = [8, 23, 45, 12, 78]

When I loop through it using a for loop, how do I access the loop index, from 1 to 5 in this case?


Source: (StackOverflow)