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Basic Python Revision Notes
HELP from Documentation
dir(module) help()
Important Characters and Sets of Characters
tab \t new line \n backslash \ string (^) " " or ' ' docstring """^ """ comparison operators == , < , > , <= , >= , != Python type boolean True , False. Logical operators not , and , or
Order of Operations
Operator Description () (^) Parentheses (grouping) f (args...) (^) Function call x [index:index] (^) Slicing x [index] (^) Subscription x.attribute (^) Attribute reference ** (^) Exponentiation
- x , - x (^) Positive, negative *, /, % (^) Multiplication, division, remainder +, - (^) Addition, subtraction in, not in, is, is not, <, <=, >, >=, <>, !=, ==
Comparisons, membership, identity
not x (^) Boolean NOT and Boolean AND or (^) Boolean OR
Variable Names
- case sensitive
- cannot start with a number (ex, 1_assd is not allowed)
Six Steps to Defining a Function
- What should your function do? Type a couple of example calls.
- Pick a meaningful name (often a verb or verb phrase): What is a short answer to "What does your function do"?
- Decide how many parameters the function takes and any return values
- Describe what your function does and any parameters and return values in the docstring
- Write the body of the function
- Test your function. Think about edge cases.
Integers and Strings
int(45) 45 int('45') 45 str(45) '45' str('45') '45' int(str(45)) 45
Calling Methods
module_name.function_name(x)
o object.method(ar guments) o dir(str) o help(str.method)
for Loops
num_vowels = 0 for char in s: if char in 'aeiouAEIOU': num_vowels = num_vowels + 1 print num_vowels vowels = '' for char in s: if char in 'aeiouAEIOU': vowels = vowels + char print vowels
Lists
Like for strings, slicing and indexing can also be used for lists
List = ['a','b',1]
length of list len(list) smallest element in list min(list) largest element in list max(list) sum of elements of list (where list items must be numeric) sum(list)
a=[1,'ab',2,'pq'] a[1][0] 'a' a[1][1] 'b' a[3][1] 'q' a[3][2]
List Methods
- append a value or string list.append('a')
- extended by another list list.extend(['a', 'b'])
a = [5] + [6] + ['a',7] print (a) [5, 6, 'a', 7]
List Mutability
We say that lists are mutable: they can be modified.
lst = [1, 2, 3] lst[0] = 'apple' lst ['appple, 2, 3]
List Aliasing
Consider the following code:
lst1 = [11, 12, 13, 14, 15, 16, 17] lst2 = lst lst1[-1] = 18 lst [11, 12, 13, 14, 15, 16, 18]
After the second statement executes, lst1 and lst2 both refer to the same list. When two
variables refer to the same objects, they are aliases. If that list is modified, both of lst1 and lst will see the change.
But be careful about:
lst1 = [11, 12, 13, 14, 15, 16, 17] lst2 = lst
def secret(s): i = 0 result = '' while s[i].isdigit(): result = result + s[i] i = i + 1 print result
secret('123') will give an error message when it runs the fourth time.
Global and Local Variables
Variables defined outside functions are global variables. Their values may be accessed inside
functions without declaration.
To modify to a global variable inside a function, the variable must be declared inside the function
using the keyword global.
def x(): global num num = 5 def y(): num = 4
num = 7 print (num) a[3][1] 'q' >>> a[3][2] List Methods - append a value or string list.append('a') - extended by another list list.extend(['a', 'b']) >>> a = [5] + [6] + ['a',7] >>> print (a) [5, 6, 'a', 7] List Mutability We say that lists are mutable: they can be modified. >>> lst = [1, 2, 3] >>> lst[0] = 'apple' >>> lst ['appple, 2, 3] List Aliasing Consider the following code: >>> lst1 = [11, 12, 13, 14, 15, 16, 17] >>> lst2 = lst >>> lst1[-1] = 18 >>> lst [11, 12, 13, 14, 15, 16, 18] After the second statement executes, lst1 and lst2 both refer to the same list. When two variables refer to the same objects, they are aliases. If that list is modified, both of lst1 and lst will see the change. But be careful about: >>> lst1 = [11, 12, 13, 14, 15, 16, 17] >>> lst2 = lst def secret(s): i = 0 result = '' while s[i].isdigit(): result = result + s[i] i = i + 1 print result >>> secret('123') will give an error message when it runs the fourth time. Global and Local Variables Variables defined outside functions are global variables. Their values may be accessed inside functions without declaration. To modify to a global variable inside a function, the variable must be declared inside the function using the keyword global. def x(): global num num = 5 def y(): num = 4 >>> num = 7 >>> print (num) 7 x() print (num) 5 y() print (num) 5
Dictionaries
The values of a dictionary can be of any type, but the keys must be of an immutable data type such
as strings, numbers, or tuples.
keys can be numbers, strings, Booleans o a list is unhashable in a dictionary (cannot be used as a key) o a tuple is hashable in a dictionary (can be used as a key). values can be dicts, strings, numbers, booleans, lists
for key in my_dict: value = my_dict[key]
This is same as:
for key, value in my_dict.items():
