Python Flashcards
Complex number in python
x = 1+2j
Left shift and arithmetic right shift operators
<<
and
>>
What does the “def” statement do?
- def statements are executed immediately, generating a function object and binding it to a name in the current module
Hashtable syntax in python
x = {}
Differences between the bitwise and boolean operators in python?
boolean are “and”, “or”, and “not”.
Common use of tuples?
Return multiple values
What do an object’s attributes include?
include both variables and functions
How to declare class in python? Constructor? add attributes?
What is interesting about the keys in python dictionaries?
Can have multiple data types for the keys in the same dictionary
Can you define two variables on one line?
It’s discouraged, but yes: f = 2.5; i = 1
What data structure is this?
evens = {2, 4, 6, 8}
A set, not a dictionary
How do you get the elements that belong to both sets? (2)
- set_1 & set_2
- set_1.intersection(set_2)
Operator for matrix product
@
What does this do?
from library import obj, obj2
From a certain module, import certain objects
What does * and ** do in a function call?
- Expands an iteratable of the args
- Expands a dictionary of the args with the names matching those in the function defintion
What are the automatic conversion to boolean rules for these values?
- numbers
- lists
- dicts
- strings
- None
- zero is false, nonzero is true
- empty is false, non-empty is true
- empty is false, non-empty is true
- empty is false, non-empty is true
- None is false, class objects are true
What are differences between python and Java? (3)
How do you apply a test to every value of an iterator, pass only those for which the test is True
How do you get the elements in a set that don’t belong to another set? (2)
This is called the difference
- odd_primes = primes - evens
- primes.difference(evens)
What’s true of python variables
they’re just pointers to objects
General syntax of list comprehensions
Check if substring exists in string
sub in s
What do we know about default args in python?
evaluated at the time the function definition is evaluated
What do mutation operators do?
For mutable objects, hanges that actual object, instead of reassigned same variable name to a new value.
x = x + y creates a new object
– x += y modifies x “in place”
What are the equality tests in python? (2)
- object identity: is
- # equal values: ==
Check if key is present in a dictionary
if key in ht:
Instantiate a set in python (empty and not empty)
s=set()
primes = {2, 3, 5, 7}
Look a function up by its name
bar = globals()[‘foo’]
bar() # invoke it
What’s true about comparisons in python?
15 < a <= 30
Describe the globals() and locals() functions
Unsure
How do you # apply a function to every value of an iterator?
Integer division operator
//
How do you concatenate a list with itself an arbitrary number of times?
Multiplication operator
l3 = 100*[‘item’]
What’s a tuple in python?
How do you instantiate them?
Immutable list
t2 = (1, 2, 3)
or
t2 = 1, 2, 3
Undefine a function
del globals()[’’]
How to check if two objects are the same object?
a is b
How do you get the elements that belong to one of two sets, but not both? (2)
- not_both = primes ^ evens
- primes.symmetric_difference(evens)
Operator for negation
-a
How do you remove a dictionary entry using its key
del ht[‘foo’]
What does *args in a function defintion do?
Can access each arg by accessing that key and val in the “kwargs” dict.
In a loop, how to how the loop?
break:
What is true about python objects?
Every object in Python has an associated hash map which stores the object’s attributes, called a “dict”
How to make a new string with a string repeated n times?
s5 = 3 * s1 #==> ‘foofoofoo’
Convert to string:
str(int) or str(float)
Negate in python (2)
Not can be before or after subject.
if not key in ht:
# or
if key not in ht:
How do you make a hash map in python? (2)
it’s the same as a dictionary.
ht = {} # empty dictionary
dct = { key1: value1, key2: value2 }
What does this do?
l3 = 100*[‘item’]
one item referenced 100 times.
If you in-place modify any
element of the list, all the others
show that modification!
How to access individual characters in a string?
Same as accessing an element in a list
Operator for unary plus
+a
What happens when an object is instantiated?
it gets a copy of the class dict
How to check if loop exited via a break
Syntax for set comprehension?
myset = { element for key in hashmap.keys() if key%2 == 0 }
In a loop, how to skip to next iteration?
continue:
Syntax for dictionary comprehension?
hashmap = { key:value for (key,value) in enumerate(iterable) }
How to concatenate strings?
Use + operator
Coerce from int to float
3 + 4.0)
How to validate that two objects are not the same object
a is not b
For loop with indexes for start and end
for i in range(BEG,END+1,STEP):
How to find the index of a substring in python?
print(s.index(sub))
Synax for lambda function
lambda arg1, arg2: arg1 + arg2
Write out the four statements you can use in error handling
What are the things that allow python to continue the same statement onto another line?
- backslash
- uncloses parentheses
Can we replace a character in a string in python?
No. Python strings are immutable. Need to create a new string.
s2[1] = 'u' #==\> ERROR s3 = s2[:1]+'u'+s2[2:] #==\> 'bur'
Bitwise not operator
~
How to iterate each character in a string
for c in “hello”:
Convert to integer:
int(string)
What does *args in a function defintion do?
Can access each arg by accessing that element in the “args” list.
Can also be used with words other than “args”. “args” is just a convention
Convert to float:
float(string)
How to find number of occurrences of substring in python?
s.count(‘c’)
How do you get the elements that belong to either of two sets? (2)
- even_or_prime = primes | evens
- primes.union(evens)
When are function definitions evaluated?
when they are encountered in the source file
What do we know about this?
from library import *
Dangerous! Public
symbols in library
will replace any
local symbols with
the same name
What’s the difference between import library as alias and from library import *
1st imports to a new namespace 2nd imports to the local namespace
Why use finally at all? Why not just put the code under and remove the indent?
Because using except keeps you in the same scope as the try block, so you can access things defined in the try
When must you use the parentheses when defining a tuple?
When you’re making a tuple with just one element
what type does 1+10.0 evaluate to?
Float
What type does 1 + ‘10’ evaluate to?
Throws error
What do we know about arithmetic on immutable types in python?
- It creates a new object even for mutation operators 2. This is interesting because this happens even though all types are just references to objects
What happens with “and” and “or” comparisons
Short circuit evaluation. Processes the conditions sequentially and if any aren’t satisfied when it’s checked, it ends evaluation of the condition
How to run something only if the loop exits without a “break” statement?
use else statement after the while
How do you raise to an exponent in python?
**
