Python 2.7 Cheat Sheet by Real Python at https://realpython.com/
1. Primitives
Numbers
Python has
integers and floats. Integers are simply whole numbers, like 314, 500,
and 716.
Floats, meanwhile, are fractional numbers like 3.14, 2.867, 76.88887.
You can
use the type method to check the value of an object.
>>>
type(3)
<type
'int'>
>>>
type(3.14)
<type
'float'>
>>>
pi = 3.14
>>>
type(pi)
<type
'float'>
In the above example, pi is the variable name, while
3.14 is the value.
You can
use the basic mathematical operators:
>>>
3+3
6
>>>
3-3
0
>>>
3/3
1
>>>
3*3
9
>>>
3**3
27
>>>
num = 3
>>>
num = num - 1
>>>
print num
2
>>>
num = num + 10
>>>
print num
12
>>>
num += 10
>>>
print num
22
>>>
num -= 12
>>>
print num
10
>>>
num *= 10
>>>
num
100
What
happens when you divide 9 by 4?
>>>
9 / 4
2
What is
the actual answer? 2 remainder 1 right? Or: 2.25
In Python
2.7x, when you divide a whole number by a whole number and the
answer is
a fractional number, Python returns a whole number without the
remainder.
In other words, this type of divison rounds the
fraction down to the
nearest
whole number (commonly known as flooring the results).
Let's look
at 12 divided by 5. If we want to get a fraction, we can just turn one of
the
numbers into a float.
>>>
12 / 5.0
2.4
>>>
12 / float(5)
2.4
There's
also a special operator called a modulus, % , that returns the remainder
after
integer division.
>>>
10 % 3
1
>>>
10 / 3
3
One common
use of modulous is determining if a number is
divisible by another
number. For
example, we know that a number is even if it's divided by 2 and the
remainder
is 0.
>>>
10 % 2
0
>>>
12 % 2
0
Finally,
make sure to use parentheses to enforce precedence.
>>>
(2 + 3) * 5
25
Strings
are used quite often in Python. Strings, are just that, a string of characters.
Strings
A
character is anything you can type on the keyboard in one keystroke, like a
letter, a
number, or a backslash.
Python
recognizes single and double quotes as the same thing, the beginning and
ends of
the strings.
>>>
“string list”
‘string
list’
>>>
‘string list’
‘string
list’
Now what
if you have a quote in the middle of the string? Python needs help to
recognize
quotes as part of the English language and not as part of the Python
language.
>>>
“I can’t do that”
“I can’t
do that”
>>>
“He said \“no\” to me”
“He
said “no” to me”
Now you
can also join strings with use of variables as well.
>>>
a = “first”
>>>
b = “last”
>>>
a + b
‘firstlast’
If you
want a space in between, you can change a to the word with a space after.
>>>
a = “first ”
>>>
a + b
“first
last”
There are
also different string methods for you to choose from as well - like
upper , lower , replace , and count .
Upper does
just what it sounds like, changes your string to have uppercase
letters.
>>>
w='woah!'
>>>
w.upper()
'WOAH!'
Lower is
just that as well, keeping your string in lower case letters.
>>>
w='WOAH!'
>>>
w.lower()
'woah!'
Replace
allows you to replace any character with another character.
>>>r='rule'
>>>r.replace('r','m')
'mule'
Count lets
you know how many times x appears in the string (can be numbers or
a string
of words as well).
>>>numbers=['1','2','1','2','2']
>>>numbers.count('2')
3
You can
also format strings with the format method.
>>>
"{0} is a lot of {1}".format("Python", "fun!")
'Python
is a lot of fun!'
Boolean
values are simply True or False .
Check to
see if a value is equal to another value with two equal signs.
Booleans
>>>
10 == 10
True
>>>
10 == 11
False
>>>
"jack" == "jack"
True
>>>
"jack" == "jake"
False
To check
for inequality use != .
>>>
10 != 10
False
>>>
10 != 11
True
>>>
"jack" != "jack"
False
>>>
"jack" != "jake"
True
You can
also test for > , < , >= , and <= .
>>>
10 > 10
False
>>>
10 < 11
True
>>>
10 >= 10
True
>>>
10 <= 11
True
>>>
10 <= 10 < 0
False
>>>
10 <= 10 < 11
True
>>>
"jack" > "jack"
False
>>>
"jack" >= "jack"
True
Lists are
containers for holding values.
>>>
fruits = ['apple','lemon','orange','grape']
>>>
fruits
['apple',
'lemon', 'orange', 'grape']
To access
the elements in the list you can use the placement in the list as an
idicator. This means numbering the items
aligned with their placement in the list.
But, you
must remember that the list starts with 0.
>>>
fruits[2]
‘orange’
If the
list is longer and you need to count from the end you can also do that.
>>>
fruits[-2]
‘orange’
Now,
sometimes lists can get long and you want to keep track of how many
elements
you have in your list. To find this, use len which is the length function.
>>>
len(fruits)
4
Use append to add a new object to the end of
the list and pop to remove
objects
from the
end.
2. Collections
Lists
>>>
fruits.append('blueberry')
>>>
fruits
['apple',
'lemon', 'orange', 'grape', 'blueberry']
>>>
fruits.append('tomato')
>>>
fruits
['apple',
'lemon', 'orange', 'grape', 'blueberry', 'tomato']
>>>
fruits.pop()
'tomato'
>>>
fruits
['apple',
'lemon', 'orange', 'grape', 'blueberry']
Check to
see if a value exists using in .
>>>
'apple' in fruits
True
>>>
'tomato' in fruits
False
A
dictionary optimizes element lookups. It uses keys and values, instead of
numbers as
placeholders. Each key must have a value. You can used a word to
look up a
value.
>>>
words={'apple':'red','lemon':'yellow'}
>>>
words
{'lemon':
'yellow', 'apple': 'red'}
>>>
words['apple']
'red'
>>>
words['lemon']
'yellow'
This will
also work with numbers.
Dictionaries
>>>
dict={'one':1,'two':2}
>>>
dict
{'two':
2, 'one': 1}
>>>
Output all
the keys as a list with keys() and all
the values with values() .
>>>
words.keys()
['lemon',
'apple']
>>>
words.values()
['yellow',
'red']
The IF
statement is used to check if a condition is true.
Essentially,
if the condition is true, the Python interpreter runs a block of
statements
called the if-block. If the statement is false, the interpreter skips the ifblock
and
processes another block of statements called the else-block. The else
clause is
optional.
Let's look
at two quick examples.
3. Control Statements
IF Statements
>>>
num = 20
>>>
if num == 20:
...
print 'the number is 20'
...
else:
...
print 'the number is not 20'
...
the
number is 20
>>>
num = 21
>>>
if num == 20:
...
print 'the number is 20'
...
else:
...
print 'the number is not 20'
...
the
number is not 20
You can
also add an elif clause to add another condition.
>>>
num = 21
>>>
if num == 20:
...
print 'the number is 20'
... elif num > 20:
...
print 'the number is greater then 20'
...
else:
...
print 'the number is less then 20'
...
the
number is greater then 20
There are
2 kinds of loops used in Python. The For loop and the While loop. For
loops are
traditionally used when you have a piece of code which you want to
repeat n
number of times. They are also commonly used to loop or iterate over
lists.
Loops
>>>
colors =('red','blue','green')
>>>
colors
('red',
'blue', 'green')
>>>
for favorite in colors:
...
print "I love " + favorite
...
I love
red
I love
blue
I love
green
While
loops, like the For Loop, are used for repeating sections of code - but unlike
a for loop, the while loop will not
run n times, but until a defined condition is met.
>>>
num = 1
>>>
num
1
>>>
while num <=5:
...
print num
...
num += 1
...
12345
Functions
are blocks of reusable code that perfrom a single
task.
You use def to define (or create) a new
function then you call a function by
adding
parameters to the function name.
4. Functions
>>>
def multiply(num1, num2):
...
return num1 * num2
...
>>>
multiply(2,2)
4
You can
also set default values for parameters.
>>>
def multiply(num1, num2=10):
...
return num1 * num2
...
>>>
multiply(2)
20