Python Language Cheat Sheet

Python is a cross-platform computer programming language. It is an object-oriented, dynamically typed language originally designed for writing automation scripts (shell). With continuous updates and new features, it is increasingly used for independent, large-scale project development.

General

• Python is case-sensitive
• Python indices start from 0
• Python uses whitespace (tabs or spaces) for indentation instead of curly braces.

Help

Modules (Libraries)

The import statement creates a new namespace and executes all statements in the associated .py file within that namespace. If you want to load module content into the current namespace, use “from module1 import *”

Scalar Types

Check data type: type(variable)

Integers

int/long - Big integers are automatically converted to long integers

Floats

float - 64-bit, no "double" type

Booleans

bool - True or False

Strings

str - Python 2.x defaults to ASCII; Python 3 is Unicode

• Strings can be single/double/triple quoted
• A string is a sequence of characters and can be treated like any other sequence
• Special characters can be escaped with \ or raw strings using r

  str1 = r'this\f?ff'
• String formatting can be achieved in several ways

  template = '%.2f %s haha $%d'
  str1 = template % (4.88, 'hola', 2)

str(), bool(), int(), and float() are also explicit type conversion functions

Null Value

NoneType(None) - Python ’null’ value (only one instance of the None object exists)

• None is not a reserved keyword, but the unique instance of “NoneType”
• None is a common default value for optional function parameters:

  def func1(a, b, c = None)
• Common usage of None:

  if variable is None :

Date and Time

datetime - Built-in Python “datetime” module provides “datetime”, “date”, “time” types.

• “datetime” combines information stored in “date” and “time”

  Create datetime from string	dt1 = datetime.strptime(‘20091031’, ‘%Y%m%d’)
  Get “date” object	dt1.date()
  Get “time” object	dt1.time()
  Format datetime to string	dt1.strftime(’%m/%d/%Y %H:%M’)
  Modify field value	dt2 = dt1.replace(minute = 0, second=30)
  Get difference	diff = dt1 - dt2 # diff is a ‘datetime.timedelta’ object

Data Structures

Tuples

A tuple is a fixed-length, immutable sequence.

Lists

A list is a variable-length, mutable sequence.

Note:

• “Start” index is inclusive, “Stop” index is exclusive.
• start/stop can be omitted, defaults to start/end.

Slicing

Sequence types include ‘str’, ‘array’, ’tuple’, ’list’, etc.

list1[start:stop]
list1[start:stop:step]
list1[::2]
str1[::-1]

Dictionaries (Hash)

Sets

A set is an unordered collection of unique elements.

Functions

• Basic form

  def func1(posArg1, keywordArg1 = 1, ..):

• “Functions are objects” common usage:

  def func1(ops = [str.strip, user_define_func, ..], ..):
      for function in ops:
          value = function(value)

• Return values

  • If no return statement, returns None.
  • Return multiple values via tuple

  return (value1, value2)
  
  value1, value2 = func1(..)

• Anonymous Functions (Lambda)

  lambda x : x * 2
  # Equivalent to:
  # def func1(x) : return x * 2

Common Functions

1. Enumerate: returns (index, value) tuples.

   for key, val in enumerate(collection):

2. Sorted: returns a new sorted list from any iterable.

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

3. Zip: groups elements from iterables into tuples.

   zip(seq1, seq2) => [('seq1_1', 'seq2_1'), (..), ..]

4. Reversed: returns a reversed iterator.

   list(reversed(range(10)))

Control Flow

1. if/else Operators:

   Check if same object	var1 is var2
   Check if different object	var1 is not var2
   Check if same value	var1 == var2

2. for Loop:

   for element in iterator :

3. pass: A null operation, used as a placeholder.

4. Ternary Expression

   value = true-expr if condition else false-expr

5. There is no switch/case; use if/elif/else.

Object-Oriented Programming

1. object is the base for all Python types.

2. Everything (numbers, strings, functions, classes, modules, etc.) is an object; every object has a ’type’. Variables are pointers to objects in memory.

3. Basic Class Structure

   class MyObject(object):
       # 'self' is equivalent to 'this' in Java/C++
       def __init__(self, name):
           self.name = name
   
       def memberFunc1(self, arg1):
           ..
   
       @staticmethod
       def classFunc2(arg1):
           ..
   
   obj1 = MyObject('name1') 
   obj1.memberFunc1('a') 
   MyObject.classFunc2('b')

4. Interactive Tools:

   dir(variable1) # List all available methods on the object

String Operations

Join list/tuple with a separator:

', '.join([ 'v1', 'v2', 'v3']) => 'v1, v2, v3'

Format string:

string1 = 'My name is {0} {name}'
newString1 = string1.format('Sean', name = 'Chen')

Split string:

sep = '-'
stringList1 = string1.split(sep)

Slice string:

start = 1
string1[start:8]

Zero-pad string:

month = '5'
month.zfill(2) => '05' 
month = '12'
month.zfill(2) => '12'

Exception Handling

1. Basic Form

try:
    ..
except ValueError as e:
    print(e)
except (TypeError, AnotherError):
    ..
except:
    ..
finally:
    ..

2. Raising Exceptions

raise AssertionError # Assertion failed
raise SystemExit # Request program exit
raise RuntimeError('Error message :..')

Comprehensions (List, Set, Dict)

Syntactic sugar for cleaner code.

1. List Comprehension:

   Creates a new list by filtering and transforming elements.

   Form:

   [expr for val in collection if condition]

   Equivalent to:

   result = []
   for val in collection:
       if condition:
           result.append(expr)

2. Dictionary Comprehension:

   {key-expr : value-expr for value in collection if condition}

3. Set Comprehension: Same as list comprehension but with {}.

4. Nested List Comprehension:

   Form:

   [expr for val in collection for innerVal in val if condition]