Python Flashcards
Why do long integers take longer to process in Python than in most other languages?
Because they are stored internally as an array of digits.
What is the difference between double quoted strings and single quoted strings in python?
There is no difference.
Are escape characters valid in Python?
Yes.
How are invalid escape characters treated in Python?
They treated like regular text.
How can escape characters be turned off in Python?
By prefacing the string with an r (to specify a raw string).
Do multi line strings require triple single-quotes or triple double-quotes?
Either.
When does the interpreter automatically add new line characters to multi line strings?
Always, unless the line ends in a backslash.
Does prefacing a multi line string in python with ‘r’ prevent the interpreter from adding new line characters?
No.
Does python provide bounds checking for when indexing strings?
Yes.
What is the output of the following?
x = “carlisle”
print(x[2:3])
r
(since slices use inclusive/exclusive format)
What is the output of the following?
x = “car”
print(x[::-1])
rac
What is the output of the following?
x = “bugman”
print(x[2:44])
gman
(valid, despite apparent out of bounds)
What is the output of the following?
x = “bugman”
x[2] = “s”
print(x)
This issues an error, since strings are immutable in Python.
Is the following valid, where x is a list of length 4?
print(x[2:99])
Yes.
How are lists in Python implemented at the memory level?
As dynamically resized arrays of pointers.
Why aren’t lists in Python implemented as linked lists at the memory level?
Because operations on linked lists (especially indexing) is much too slow.
What is the asymptotic complexity of accessing / updating a list element by index?
O(1)
What is the asymptotic complexity of appending to a list?
O(1) if space is available, O(n) otherwise.
What is the asymptotic complexity of inserting into a list?
O(n)
What is the asymptotic complexity of building a list by inserting every element into the first position?
O(n^2)
How can you attempt to access a key’s value in a dictionary, without raising a KeyError exception should that key not exist?
By using dict.get(key), over dict[key].
What is the (amortized) cost of a lookup in a dictionary?
O(1), as there is a constant number of steps in executing the hash function.
How are tuples of 1 element created?
With a trailing comma.
What are the 2 primary differences between sequences and sets in python?
Unlike sequences, sets are unordered and do not allow duplicates.
How do you initialize an empty set in python?
Using the set() function, since {} creates an empty dictionary.
How do sets prevent duplicates?
By being internally implemented as a hash table.
Does adding an element that already exists to a set cause an error?
No, the duplicate value is simply not inserted.
Why must set elements be immutable in python?
Because they must be hashable, as the underlying data structure is a hash table.
4 spaces can always be used in place of tab characters for delimiting coding blocks.
It is generally recommended to use 4 spaces over a tab, but never use both techniques in the same source file.
Which data types can follow the “in” keyword?
Sequences.
A value of NULL is assigned to all uninitialized variables.
False. Variables meant to be defined later must be explicitly set equal to ‘None,’ which is Python’s equivalent to Null.
Functions return void by default.
False, they return None by default.
When are module imports handled in Python, and when are include directives handled in C?
In C, extended code is converted into object code at compile time, whereas the Python import process occurs at run time.
For what files does the Python interpreter create .pyc files?
Only for imported modules, and not for the main script.
A module may be imported more than once.
A module may be subject to multiple import statements, but the interpreter will not physically import a module more than once.
Which environment allows developers to add their own folders to the module search path?
PYTHONPATH
(similar to Java’s CLASSPATH)
How do Python global variables and C global variables differ?
Python global variables aren’t global to all modules, unless imported.
When can you not use the ‘global’ keyword to access a global variable from within a function?
If a local variable of the same name has already been declared.
If mod_name is the name of a module, when does the following code fail?
dir(mod_name)
When mod_name hasn’t been imported.
When is the __name__ global variable not accurately set to the name of the current module?
When the current module represents the starting module for the application, in which case __name__ is set to “__main__”.
How can a package be defined such that only 1 of its modules it imported when “from package import *” is called.
__all__ = [“module_name”] must be included in the packages __init__.py file.
