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Python Interview Questions and Answers

1) What is Python? What are the benefits of using Python?

Python is a programming language with objects, modules, threads, exceptions and automatic memory management. The benefits of pythons are that it is simple and easy, portable, extensible, build-in data structure and it is an open source.

2)  What is PEP 8?

PEP 8 is a coding convention, a set of recommendation, about how to write your Python code more readable.

3) What is pickling and unpickling?

Pickle module accepts any Python object and converts it into a string representation and dumps it into a file by using dump function, this process is called pickling.  While the process of retrieving original Python objects from the stored string representation is called unpickling.

4) How Python is interpreted?

Python language is an interpreted language. Python program runs directly from the source code. It converts the source code that is written by the programmer into an intermediate language, which is again translated into machine language that has to be executed.

5) How memory is managed in Python?

– Python memory is managed by Python private heap space. All Python objects and data structures are located in a private heap. The programmer does not have an access to this private heap and interpreter takes care of this Python private heap.

– The allocation of Python heap space for Python objects is done by Python memory manager.  The core API gives access to some tools for the programmer to code.

– Python also have an inbuilt garbage collector, which recycle all the unused memory and frees the memory and makes it available to the heap space.

6) What are the tools that help to find bugs or perform static analysis?

PyChecker is a static analysis tool that detects the bugs in Python source code and warns about the style and complexity of the bug. Pylint is another tool that verifies whether the module meets the coding standard.

7) What are Python decorators?

A Python decorator is a specific change that we make in Python syntax to alter functions easily.

8) What is the difference between list and tuple?

The difference between list and tuple is that list is mutable while tuple is not. Tuple can be hashed for e.g as a key for dictionaries.

9) How are arguments passed by value or by reference?

Everything in Python is an object and all variables hold references to the objects. The references values are according to the functions; as a result you cannot change the value of the references. However, you can change the objects if it is mutable.

10) What is Dict and List comprehensions are?

They are syntax constructions to ease the creation of a Dictionary or List based on existing iterable.

11) What are the built-in type does python provides?

There are mutable and Immutable types of Pythons built in types Mutable built-in types

– List

– Sets

– Dictionaries

Immutable built-in types

– Strings

– Tuples

– Numbers

12) What is namespace in Python?

In Python, every name introduced has a place where it lives and can be hooked for. This is known as namespace. It is like a box where a variable name is mapped to the object placed. Whenever the variable is searched out, this box will be searched, to get corresponding object.

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13) What is lambda in Python?

It is a single expression anonymous function often used as inline function.

14) Why lambda forms in python does not have statements?

A lambda form in python does not have statements as it is used to make new function object and then return them at runtime.

15) What is pass in Python?

Pass means, no-operation Python statement, or in other words it is a place holder in compound statement, where there should be a blank left and nothing has to be written there.

16) In Python what are iterators?

In Python, iterators are used to iterate a group of elements, containers like list.

17) What is unittest in Python?

A unit testing framework in Python is known as unittest.  It supports sharing of setups, automation testing, shutdown code for tests, aggregation of tests into collections etc.

18) In Python what is slicing?

A mechanism to select a range of items from sequence types like list, tuple, strings etc. is known as slicing.

19) What are generators in Python?

The way of implementing iterators are known as generators.  It is a normal function except that it yields expression in the function.

20) What is docstring in Python?

A Python documentation string is known as docstring, it is a way of documenting Python functions, modules and classes.

21)  How can you copy an object in Python?

To copy an object in Python, you can try copy.copy () or copy.deepcopy() for the general case. You cannot copy all objects but most of them.

22) What is negative index in Python?

Python sequences can be index in positive and negative numbers.   For positive index, 0 is the first index, 1 is the second index and so forth.  For negative index, (-1) is the last index and (-2) is the second last index and so forth.

23) How you can convert a number to a string?

In order to convert a number into a string, use the inbuilt function str().  If you want a octal or hexadecimal representation, use the inbuilt function oct() or hex().

24) What is the difference between Xrange and range?

Xrange returns the xrange object while range returns the list, and uses the same memory and no matter what the range size is.

25) What is module and package in Python?

In Python, module is the way to structure program. Each Python program file is a module, which imports other modules like objects and attributes.

The folder of Python program is a package of modules.  A package can have modules or subfolders.

26) How can I find the methods or attributes of an object?

For an instance x of a user-defined class, dir(x) returns an alphabetized list of the names containing the instance attributes and methods and attributes defined by its class.

27) Is there an equivalent of C’s “?:” ternary operator?

No.

28) How do I convert a number to a string?

To convert, e.g., the number 144 to the string ‘144’, use the built-in function str(). If you want a hexadecimal or octal representation, use the built-in functions hex() or oct(). For fancy formatting, use the % operator on strings, e.g. “%04d” % 144 yields ‘0144’ and “%.3f” % (1/3.0) yields ‘0.333’. See the library reference manual for details.

29) How do I modify a string in place?

You can’t, because strings are immutable. If you need an object with this ability, try converting the string to a list or use the array module:

>>> s = “Hello, world”
>>> a = list(s)
>>>print a
[‘H’, ‘e’, ‘l’, ‘l’, ‘o’, ‘,’, ‘ ‘, ‘w’, ‘o’, ‘r’, ‘l’, ‘d’]
>>> a[7:] = list(“there!”)
>>>”.join(a)
‘Hello, there!’

>>> import array
>>> a = array.array(‘c’, s)
>>> print a
array(‘c’, ‘Hello, world’)
>>> a[0] = ‘y’ ; print a
array(‘c’, ‘yello world’)
>>> a.tostring()
‘yello, world’

30) How do I use strings to call functions/methods?

There are various techniques.

* The best is to use a dictionary that maps strings to functions. The primary advantage of this technique is that the strings do not need to match the names of the functions. This is also the primary technique used to emulate a case construct:

def a():
pass

def b():
pass

dispatch = {‘go’: a, ‘stop’: b} # Note lack of parens for funcs

dispatch[get_input()]() # Note trailing parens to call function
*
Use the built-in function getattr():

import foo
getattr(foo, ‘bar’)()

Note that getattr() works on any object, including classes, class instances, modules, and so on.

This is used in several places in the standard library, like this:

class Foo:
def do_foo(self):

def do_bar(self):

f = getattr(foo_instance, ‘do_’ + opname)
f()

*
Use locals() or eval() to resolve the function name:

def myFunc():
print “hello”

fname = “myFunc”

f = locals()[fname]
f()

f = eval(fname)
f()

Note: Using eval() is slow and dangerous. If you don’t have absolute control over the contents of the string, someone could pass a string that resulted in an arbitrary function being executed.

31) How do I convert between tuples and lists?

The function tuple(seq) converts any sequence(actually, any iterable) into a tuple with the same items in the same order.

For example, tuple([1, 2, 3]) yields (1, 2, 3) and tuple(‘abc’) yields (‘a’, ‘b’, ‘c’). If the argument is a tuple, it does not make a copy but returns the same object, so it is cheap to call tuple() when you aren’t sure that an object is already a tuple.

The function list(seq) converts any sequence or iterable into a list with the same items in the same order. For example, list((1, 2, 3)) yields [1, 2, 3] and list(‘abc’) yields [‘a’, ‘b’, ‘c’]. If the argument is a list, it makes a copy just like seq[:] would.

32) What’s a negative index?

Python sequences are indexed with positive numbers and negative numbers. For positive numbers 0 is the first index 1 is the second index and so forth. For negative indices -1 is the last index and -2 is the penultimate (next to last) index and so forth. Think of seq[-n] as the same as seq[len(seq)-n].

Using negative indices can be very convenient. For example S[:-1] is all of the string except for its last character, which is useful for removing the trailing newline from a string.

33) How do you remove duplicates from a list?

If you don’t mind reordering the list, sort it and then scan from the end of the list, deleting duplicates as you go:

if List:
List.sort()
last = List[-1]
for i in range(len(List)-2, -1, -1):
if last==List[i]: del List[i]
else: last=List[i]

If all elements of the list may be used as dictionary keys (i.e. they are all hash able) this is often faster

d = {}
for x in List: d[x]=x
List = d.values()

34) How do you make an array in Python?

Use a list:
[“this”, 1, “is”, “an”, “array”]

Lists are equivalent to C or Pascal arrays in their time complexity; the primary difference is that a Python list can contain objects of many different types.

The array module also provides methods for creating arrays of fixed types with compact representations, but they are slower to index than lists. Also note that the Numeric extensions and others define array-like structures with various characteristics as well.

To get Lisp-style linked lists, you can emulate cons cells using tuples:

lisp_list = (“like”, (“this”, (“example”, None) ) )

If mutability is desired, you could use lists instead of tuples. Here the analogue of lisp car is lisp_list[0] and the analogue of cdr is lisp_list[1]. Only do this if you’re sure you really need to, because it’s usually a lot slower than using Python lists.

35) How do I apply a method to a sequence of objects?

Use a list comprehension:

result = [obj.method() for obj in List]

More generically, you can try the following function:

def method_map(objects, method, arguments):
“””method_map([a,b], “meth”, (1,2)) gives [a.meth(1,2), b.meth(1,2)]”””
nobjects = len(objects)
methods = map(getattr, objects, [method]*nobjects)
return map(apply, methods, [arguments]*nobjects)

36) What is self?

Self is merely a conventional name for the first argument of a method. A method defined as meth(self, a, b, c) should be called as x.meth(a, b, c) for some instance x of the class in which the definition occurs; the called method will think it is called as meth(x, a, b, c).

37) How do I call a method defined in a base class from a derived class that overrides it?

If you’re using new-style classes, use the built-in super() function:

class Derived(Base):
def meth (self):
super(Derived, self).meth()

If you’re using classic classes: For a class definition such as class Derived(Base): … you can call method meth() defined in Base (or one of Base’s base classes) as Base.meth(self, arguments…). Here, Base.meth is an unbound method, so you need to provide the self argument.

38) How can I organize my code to make it easier to change the base class?

You could define an alias for the base class, assign the real base class to it before your class definition, and use the alias throughout your class. Then all you have to change is the value assigned to the alias. Incidentally, this trick is also handy if you want to decide dynamically (e.g. depending on availability of resources) which base class to use. Example:

BaseAlias = <real base class>
class Derived(BaseAlias):
def meth(self):
BaseAlias.meth(self)

39) Where is the math.py (socket.py, regex.py, etc.) source file?

There are (at least) three kinds of modules in Python:

1. modules written in Python (.py);
2. modules written in C and dynamically loaded (.dll, .pyd, .so, .sl, etc);
3. modules written in C and linked with the interpreter; to get a list of these, type:
import sys
print sys.builtin_module_names

40) Why don’t my signal handlers work?

The most common problem is that the signal handler is declared with the wrong argument list. It is called as

handler(signum, frame)

so it should be declared with two arguments:

def handler(signum, frame):

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