Python Cheat Sheet Just The Basics - Arianne Colton And Sean Chen (Page 2 of 2) in pdf

-o

bject

rienteD

unctions

xception

anDlinG

roGramminG

Python is pass by reference, function arguments

1. Basic Form :

• Application :

are passed by reference.

1. 'object' is the root of all Python types

try:

sorted(set('abc

bcd')) => [' ',

• Basic Form :

2. Everything (number, string, function, class, module,

'a', 'b', 'c', 'd']

except

ValueError

as e:

etc.) is an object, each object has a 'type'. Object

# returns sorted unique characters

def func1(posArg1,

keywordArg1

variable is a pointer to its location in memory.

except (TypeError, AnotherError):

..):

3. Zip pairs up elements of a number of lists, tuples or

3. All objects are reference-counted.

except:

other sequences to create a list of tuples :

Note :

sys.getrefcount(5)

finally:

zip(seq1, seq2) =>

• Keyword arguments MUST follow positional

# clean up, e.g. close db

= 5,

arguments.

[('seq1_1', 'seq2_1'), (..), ..]

# This creates a 'reference' to the object on the

2. Raise Exception Manually

• Python by default is NOT "lazy evaluation",

right side of =, thus both a and b point to 5

• Zip can take arbitrary number of sequences.

expressions are evaluated immediately.

raise AssertionError # assertion failed

However, the number of elements it produces is

sys.getrefcount(5)

# request program exit

raise SystemExit

determined by the 'shortest' sequence.

• Function Call Mechanism :

raise

RuntimeError('Error message :

del(a); sys.getrefcount(5) =>

• Application : Simultaneously iterating over multiple

..')

1. All functions are local to the module level

sequences :

scope. See 'Module' section.

4. Class Basic Form :

2. Internally, arguments are packed into a tuple

for i, (a, b) in

, s

class MyObject(object):

ist

et anD

ict

and dict, function receives a tuple 'args' and

enumerate(zip(seq1, seq2)):

# 'self' is equivalent of 'this' in Java/C++

dict 'kwargs' and internally unpack.

def __init__(self, name):

omprehansions

• Unzip - another way to think about this is

• Common usage of 'Functions are objects' :

self.name

name

converting a list of rows to a list of columns.

def

func1(ops

= [str.strip,

user_

Syntactic sugar that makes code easier to read and write

def memberFunc1(self, arg1):

define_func, ..], ..):

seq1,

seq2

= zip(*zipOutput)

1. List comprehensions

for

function

in ops:

value

= function(value)

@staticmethod

• Concisely form a new list by filtering the elements

4. Reversed iterates over the elements of a sequence

def classFunc2(arg1):

of a collection and transforming the elements

in reverse order.

RETURN VALUES

passing the filter in one concise expression.

• Basic form :

• None is returned if end of function is reached

obj1

= MyObject('name1')

list(reversed(range(10)))

without encountering a return statement.

obj1.memberFunc1('a')

[expr

for

val

collection

if condition]

MyObject.classFunc2('b')

• Multiple values return via ONE tuple object

* reversed() returns the iterator,

makes

list()

A shortcut for :

it a list.

5. Useful interactive tool :

return (value1, value2)

result = []

value1,

value2

= func1(..)

for

val

in collection:

# list all methods available on

dir(variable1)

the object

if condition:

ontrol anD

low

ANONYMOUS

(AKA LAMBDA) FUNCTIONS

result.append(expr)

• What is Anonymous function?

The filter condition can be omitted, leaving only the

1. Operators for conditions in 'if else' :

A simple function consisting of a single statement.

expression.

ommon

trinG

lambda

Check if two variables are

2. Dict Comprehension

var1

var2

operations

same object

# def func1(x) : return x * 2

• Basic form :

. . . are different object

var1

is not

var2

• Application of lambda functions : 'curring' aka

{key-expr

value-expr

for

value

Concatenate

Check if two variables have

', '.join([ 'v1', 'v2',

var1

var2

deriving new functions from existing ones by

List/Tuple with

collection

if condition}

same value

'v3']) =>

'v1, v2, v3'

Separator

partial argument application.

3. Set Comprehension

WARNING : Use 'and', 'or', 'not' operators for

string1

'My name is

{0}

• Basic form : same as List Comprehension except

ma60

= lambda

: pd.rolling_mean(x,

{name}'

compound conditions, not &&, ||, !.

with curly braces instead of

60)

[]

Format String

newString1

= string1.

2. Common usage of 'for' operator :

format('Sean',

name

4. Nested list Comprehensions

USEFUL FUNCTIONS

(FOR DATA STRUCTURES)

'Chen')

Iterating over a collection (i.e. list

for

element

• Basic form :

1. Enumerate returns a sequence (i, value) tuples

or tuple) or an iterator

iterator :

sep = '-';

where i is the index of current item.

. . . If elements are sequences,

for a, b,

[expr

for

val

collection

for

stringList1

Split String

can be 'unpack'

iterator :

innerVal

val

if condition]

string1.split(sep)

for i,

value

in enumerate(collection):

3. 'pass' - no-op statement. Used in blocks where no

action is to be taken.

• Application : Create a dict mapping of value

Get Substring start = 1;

string1[start:8]

Created by Arianne Colton and Sean Chen

of a sequence (assumed to be unique) to their

4. Ternary Expression - aka less verbose 'if else'

locations in the sequence.

• Basic Form :

month

= '5';

2. Sorted returns a new sorted list from any sequence

month.zfill(2) => '05'

String Padding

Based on content from

value

true-expr

condition

with Zeros

else

false-expr

month

= '12';

'Python for Data Analysis' by Wes McKinney

sorted([2, 1, 3]) => [1, 2, 3]

month.zfill(2) => '12'

5. No switch/case statement, use if/elif instead.

Updated: May 3, 2016

Python Cheat Sheet Just The Basics - Arianne Colton And Sean Chen Page 2

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