Python Interview Questions and Answers

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:
<do something with x>

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]
<do something with x>

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):
<do something with x>

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]:
<do something with x>

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