Python Interview Questions And Answers

Image via WikipediaHow can my code discover the name of an object?
Generally speaking, it can't, because objects don't really have names. Essentially, assignment always binds a name to a value; The same is true of def and class statements, but in that case the value is a callable. Consider the following code:

class A:
pass

B = A

a = B()
b = a
print b
<__main__.A instance at 016D07CC>
print a
<__main__.A instance at 016D07CC>

Arguably the class has a name: even though it is bound to two names and invoked through the name B the created instance is still reported as an instance of class A. However, it is impossible to say whether the instance's name is a or b, since both names are bound to the same value.

Generally speaking it should not be necessary for your code to "know the names" of particular values. Unless you are deliberately writing introspective programs, this is usually an indication that a change of approach might be beneficial.

In comp.lang.python, Fredrik Lundh once gave an excellent analogy in answer to this question:

The same way as you get the name of that cat you found on your porch: the cat (object) itself cannot tell you its name, and it doesn't really care -- so the only way to find out what it's called is to ask all your neighbours (namespaces) if it's their cat (object)...

....and don't be surprised if you'll find that it's known by many names, or no name at all!
Is there an equivalent of C's "?:" ternary operator?
No.
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.
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'
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.
Is there an equivalent to Perl's chomp() for removing trailing newlines from strings?
Starting with Python 2.2, you can use S.rstrip("\r\n") to remove all occurences of any line terminator from the end of the string S without removing other trailing whitespace. If the string S represents more than one line, with several empty lines at the end, the line terminators for all the blank lines will be removed:

>>> lines = ("line 1 \r\n"
... "\r\n"
... "\r\n")
>>> lines.rstrip("\n\r")
"line 1 "

Since this is typically only desired when reading text one line at a time, using S.rstrip() this way works well.

For older versions of Python, There are two partial substitutes:

* If you want to remove all trailing whitespace, use the rstrip() method of string objects. This removes all trailing whitespace, not just a single newline.
* Otherwise, if there is only one line in the string S, use S.splitlines()[0].
Is there a scanf() or sscanf() equivalent?
Not as such.

For simple input parsing, the easiest approach is usually to split the line into whitespace-delimited words using the split() method of string objects and then convert decimal strings to numeric values using int() or float(). split() supports an optional "sep" parameter which is useful if the line uses something other than whitespace as a separator.

For more complicated input parsing, regular expressions more powerful than C's sscanf() and better suited for the task.
Is there a scanf() or sscanf() equivalent?

Not as such.

For simple input parsing, the easiest approach is usually to split the line into whitespace-delimited words using the split() method of string objects and then convert decimal strings to numeric values using int() or float(). split() supports an optional "sep" parameter which is useful if the line uses something other than whitespace as a separator.

For more complicated input parsing, regular expressions more powerful than C's sscanf() and better suited for the task. 1.3.9 What does 'UnicodeError: ASCII [decoding,encoding] error: ordinal not in range(128)' mean?

This error indicates that your Python installation can handle only 7-bit ASCII strings. There are a couple ways to fix or work around the problem.

If your programs must handle data in arbitrary character set encodings, the environment the application runs in will generally identify the encoding of the data it is handing you. You need to convert the input to Unicode data using that encoding. For example, a program that handles email or web input will typically find character set encoding information in Content-Type headers. This can then be used to properly convert input data to Unicode. Assuming the string referred to by value is encoded as UTF-8:

value = unicode(value, "utf-8")

will return a Unicode object. If the data is not correctly encoded as UTF-8, the above call will raise a UnicodeError exception.

If you only want strings converted to Unicode which have non-ASCII data, you can try converting them first assuming an ASCII encoding, and then generate Unicode objects if that fails:

try:
x = unicode(value, "ascii")
except UnicodeError:
value = unicode(value, "utf-8")
else:
# value was valid ASCII data
pass

It's possible to set a default encoding in a file called sitecustomize.py that's part of the Python library. However, this isn't recommended because changing the Python-wide default encoding may cause third-party extension modules to fail.

Note that on Windows, there is an encoding known as "mbcs", which uses an encoding specific to your current locale. In many cases, and particularly when working with COM, this may be an appropriate default encoding to use.
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.
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.
How do I iterate over a sequence in reverse order?
If it is a list, the fastest solution is

list.reverse()
try:
for x in list:
"do something with x"
finally:
list.reverse()

This has the disadvantage that while you are in the loop, the list is temporarily reversed. If you don't like this, you can make a copy. This appears expensive but is actually faster than other solutions:

rev = list[:]
rev.reverse()
for x in rev:


If it's not a list, a more general but slower solution is:

for i in range(len(sequence)-1, -1, -1):
x = sequence[i]


A more elegant solution, is to define a class which acts as a sequence and yields the elements in reverse order (solution due to Steve Majewski):

class Rev:
def __init__(self, seq):
self.forw = seq
def __len__(self):
return len(self.forw)
def __getitem__(self, i):
return self.forw[-(i + 1)]

You can now simply write:

for x in Rev(list):


Unfortunately, this solution is slowest of all, due to the method call overhead.

With Python 2.3, you can use an extended slice syntax:

for x in sequence[::-1]:

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