PCAP sample questions

Question 1

After the statement `import math as m` runs, which name refers to the module inside the importing file?

Accepted Answer

m only, because the alias replaces the original module name in this namespace. `import x as y` binds exactly one name, y, in the importing namespace, so only m is defined; referring to math afterwards raises NameError. The form that binds both names does not exist: you would need a second plain `import math` statement for that.

Question 2

When does the finally branch of a try statement run?

Accepted Answer

Always, whether the try branch succeeded, raised a handled exception, or raised an unhandled one. finally is the clean-up branch: the interpreter runs it on every way out of the try statement, including a return, a break, or an exception that is still propagating. The branch that runs only when nothing was raised is else, which is what "only when the try branch completed without raising anything" describes.

Question 3

A settings module defines and raises its own exception:

class ConfigError(Exception):
    def __init__(self, key, message='missing key'):
        super().__init__(message)
        self.key = key

try:
    raise ConfigError('timeout')
except ConfigError as err:
    print(err.key, err.args[0])

What is printed?

Accepted Answer

timeout missing key. The custom constructor stores the key on the instance and passes the message up to the base constructor, which is what fills args, so both values are available in the handler. If the call to super().__init__ were left out, args would stay empty and err.args[0] would raise IndexError.

Question 4

An input validator uses character-class methods:

print('abc123'.isalnum(), 'abc 123'.isalnum(), '123'.isdigit(), ''.isdigit())

What does this code print?

Accepted Answer

True False True False. isalnum() is True only if every character is a letter or digit, so the space makes 'abc 123' fail. isdigit() is True for '123', but isdigit(), like isalnum(), isalpha() and isspace(), returns False for an empty string because it requires at least one character. Expecting True for '' is the trap: 'all characters are digits' is not treated as vacuously true.

Question 5

split() and join() are chained:

print('a1b2c3'.split('b'), 'x'.join('ab'.split('a')))

What does this code print?

Accepted Answer

['a1', '2c3'] xb. split('b') cuts at the separator and drops it, giving ['a1', '2c3']. 'ab'.split('a') returns ['', 'b'] because the separator at position 0 leaves an empty string before it; joining those two items with 'x' gives 'x' + 'b'. The answer 'b' forgets the leading empty string that split() keeps when the text starts with the separator.

Question 6

Type checks are made on a subclass object:

class A:
    pass

class B(A):
    pass

b = B()
print(isinstance(b, A), isinstance(A(), B), type(b) == A)

What does this code print?

Accepted Answer

True False False. isinstance() accepts subclass instances, so a B object is an A. The reverse fails because an A object is not a B. type(b) returns exactly B and ignores inheritance, so comparing it with A gives False; that difference is why isinstance() is preferred for type checks.

Question 7

A three-level hierarchy is inspected:

class A:
    pass

class B(A):
    pass

class C(B):
    pass

print([k.__name__ for k in C.__bases__], issubclass(C, A))

What does this code print?

Accepted Answer

['B'] True. __bases__ lists only the direct superclasses written in the class statement, so C.__bases__ contains B alone. issubclass() follows the whole chain, however, so C is still a subclass of A. Listing A or object in __bases__ confuses it with the full method resolution order.

Question 8

A class uses a different name for its first parameter:

class A:
    def __init__(me, v):
        me.v = v

def get(me):
        return me.v * 2

print(A(4).get())

What does this code print?

Accepted Answer

8. The first parameter of a method receives the instance whatever it is called; self is a strong convention, not a keyword. me.v stores 4 and get() returns 8. Nothing in the language checks the parameter name, so no TypeError or NameError occurs.

Question 9

A comprehension filters and transforms a range:

print([x * x for x in range(5) if x % 2])

What does this code print?

Accepted Answer

[1, 9]. x % 2 is truthy (1) for odd numbers, so only 1 and 3 pass the filter and are squared. range(5) stops at 4, so 5 never appears and 25 is not produced. '[0, 4, 16]' reads the condition as 'x is even', but a zero remainder is falsy.

Question 10

map() is given two iterables of different lengths:

print(list(map(lambda a, b: a * b, [1, 2, 3], [4, 5])))

What does this code print?

Accepted Answer

[4, 10]. With several iterables, map() calls the function with one item from each and stops as soon as the shortest iterable is exhausted, so only two products are produced. It neither pads the shorter list nor raises an error by default.

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PCAP sample questions

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