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Use dir(variable) to list all available methods for any object
Object Types & Type System
Built-in Object Type Overview
Object Type
Examples
Immutable?
Notes
Numbers
1234, 3.1415, 3+4j, 0b111, Decimal(), Fraction()
✓
Unlimited precision integers
Strings
'spam', "bos's", b'a\x01c', u'sp\xc4m'
✓
Sequences of text
Lists
[1, [2, 'three'], 4.5], list(range(10))
✗
Ordered, mutable sequences
Tuples
(1, 'spam', 4, 'U'), tuple('spam'), namedtuple
✓
Fixed, immutable sequences
Dictionaries
{'food': 'spam', 'taste': 'yum'}, dict(hours=10)
✗
Key-value mappings
Sets
set('abc'), {'a', 'b', 'c'}
✗
Unordered unique collections
Files
open('eggs.txt'), open(r'C:\ham.bin', 'wb')
—
I/O streams
Booleans
True, False
✓
Truth values
Program Units
functions, modules, classes
—
Code organization
Key Concepts:
Immutability: Numbers, Strings, Tuples cannot be modified after creation
Mutability: Lists, Dictionaries, Sets are mutable (can be changed in-place)
Type Checking (Use Sparingly!)
# Bad practice - avoids polymorphismtype(L) ==list# Better - use isinstance for type checkingisinstance(L, list)
isinstance(x, (list, tuple)) # Check multiple types# Best - use duck typing (Pythonic way)# Don't ask "what is it?", ask "what can it do?"
Numeric Types
Numeric Literals
Literal
Interpretation
1234, 24
Integers (unlimited size)
1.23, 1.3e-10, 4E210
Floating-point numbers
0o117, 0x9ff, 0b10101
Octal, hexadecimal, binary
3+4j, 3.0+4.0j, 3J
Complex numbers
set('spam'), {1, 2, 3, 4}
Sets
Decimal('1.0'), Fraction(1, 3)
Decimal and fraction extensions
bool(x), True, False
Boolean type and constants
Numeric Display & Representation
# Precision vs. user-friendly display3.1415*2# Full precision: 6.28300000004print(3.1415*2) # User-friendly: 6.283# Formatted output'%e'%num# Scientific: '3.333e-01''%4.2f'%num# Fixed-point: '0.33''{0:4.2f}'.format(num) # Format method: '0.33'
Numeric Operations
# Built-in numeric functionspow(x, y) # Power: x^yabs(x) # Absolute valueround(x, ndigits) # Round to n decimalsint(x) # Convert to int (truncates float)float(x) # Convert to floathex(x) # Convert to hex string: '0xf'oct(x) # Convert to octal string: '0o17'bin(x) # Convert to binary string: '0b1111'# Conversion with baseint('0xf', 16) # Parse hex: 15int('0o17', 8) # Parse octal: 15
Math Module (Standard Library)
importmath# Constantsmath.pi# 3.141592...math.e# 2.71828...# Functionsmath.sqrt(x) # Square rootmath.sin(x), math.cos(x), math.tan(x) # Trigonometrymath.floor(x) # Round towards negative infinitymath.ceil(x) # Round towards positive infinitymath.trunc(x) # Discard fractional partmath.min(), math.max() # Min and max functions
Random Module (Standard Library)
importrandomrandom.random() # Random float [0.0, 1.0)random.randint(a, b) # Random integer [a, b]random.choice(sequence) # Random element from sequencerandom.shuffle(list) # Shuffle list in-place
# Python allows chained comparisonsX<Y<Z# True (equivalent to: X < Y AND Y < Z)X<Y>Z# Works as expected1<2<3.0<4# True1==2<3# Same as: 1 == 2 AND 2 < 3# Warning: Floating-point comparisons1.1+2.2==3.3# May not be True due to rounding# 1.1 + 2.2 = 3.3000000000000003int(1.1+2.2) ==3# This works
S='Spam'# Length and accesslen(S) # 4S[0] # 'S' (first)S[-1] # 'm' (last)S[-2] # 'a' (second from end)# SlicingS[1:3] # 'pa' (excludes end index)S[0:3] # 'Spa'S[1:] # 'pam' (to end)S[:-1] # 'Spa' (everything but last)S[::2] # 'Sm' (every other character)S[::-1] # 'mapS' (reversed)# OperationsS+'xyz'# Concatenation: 'Spamxyz'S*3# Repetition: 'SpamSpamSpam''ab'in'cabsf'# Membership: True
String Slicing & Indexing Visualization
Index: 0 1 2 3 4 5 6 7
| S | L | I | C | E | S | B | C |
Index: -8 -7 -6 -5 -4 -3 -2 -1
String Methods
Method
Example
Output
Notes
find
S.find('pa')
1
Return index or -1 if not found
replace
S.replace('pa', 'XYZ')
'SXYZm'
Strings are immutable
split
'a,b,c'.split(',')
['a', 'b', 'c']
Split by delimiter
upper
S.upper()
'SPAM'
Convert to uppercase
lower
S.lower()
'spam'
Convert to lowercase
strip
' spam '.strip()
'spam'
Remove leading/trailing whitespace
rstrip
'spam\n'.rstrip()
'spam'
Remove trailing chars (default: whitespace)
lstrip
' spam'.lstrip()
'spam'
Remove leading chars
isalpha
S.isalpha()
True/False
All alphabetic?
isdigit
'123'.isdigit()
True
All digits?
startswith
S.startswith('S')
True/False
Starts with substring?
endswith
S.endswith('m')
True/False
Ends with substring?
join
','.join(['a', 'b'])
'a,b'
Join iterable with delimiter
count
'aaa'.count('a')
3
Count occurrences
encode
'spam'.encode('utf8')
b'spam'
Encode to bytes
String Immutability
S='spam'S[0] ='z'# Error! Strings are immutable# Correct approachS='z'+S[1:] # Create new string: 'zpam'
String Formatting
Old-style % formatting
'%s, %s'% ('eggs', 'SPAM') # 'eggs, SPAM''There are %d %s'% (2, 'birds') # 'There are 2 birds'# Formatting with dictionary'%(qty)d more %(food)s'% {'qty': 1, 'food': 'spam'} # '1 more spam'
Method-style .format() — Python 3.0+
# By position'{0},{1},{2}'.format('spam', 'ham', 'eggs') # 'spam,ham,eggs'# By keyword'{motto},{pork},{food}'.format(motto='spam', pork='ham', food='eggs')
# Mixed'{motto},{0},{food}'.format('ham', motto='spam', food='egg') # 'spam,ham,egg'# Relative position (no indices)'{},{},{}'.format('spam', 'ham', 'eggs')
# Accessing attributes'My {0.platform}'.format(sys) # 'My win32''My {map[kind]}'.format(map={'kind': 'laptop'}) # 'My laptop'
F-strings — Python 3.6+
name, age='Bob', 40f'Hello {name}, age {age}'# f'Hello Bob, age 40'f'{name.upper()}'# f'BOB'f'{2**8}'# f'256'f'{value:.2f}'# f-string formatting
Special String Types
Raw strings — Backslashes not interpreted
myfile=open(r'C:\new\text.dat', 'w') # \n is not interpreted as newline
Triple-quoted strings — Multi-line text
mantra="""Always lookon the brightside of life."""# Common uses:# 1. Docstrings for functions/classes# 2. Temporarily disable code (act as block comments)# 3. Embed quotes easilyX=1"""import osprint(os.getcwd())"""Y=2# Code in triple quotes is ignored
Byte strings — Python 3.0+
b'a\x01c'# Byte literals (ASCII only)'spam'.encode('utf8') # Encode string to bytes: b'spam'b'spam'.decode('utf8') # Decode bytes to string: 'spam'
# Read entire filetext=f.read() # Single string (NOT one line!)lines=f.readlines() # List of lines (with \n)# Read line by lineline=f.readline() # Single line with \nchar=f.read(N) # N characters# Iterate through linesforlineinopen('myfile.txt'):
print(line, end='')
# Reset positionf.seek(0) # Go to beginningf.tell() # Current position
Writing Files
f.write(string) # Write stringf.writelines(list) # Write list of stringsf.flush() # Force write to disk (buffering)f.seek(offset) # Change position for next operationf.truncate([size]) # Truncate to size
⚠️Note: Output files are buffered. Use flush() or close() to ensure data is written.
File Context Manager
# Safe pattern - auto-closesmyfile=open(r'~/data.txt', 'r')
try:
forlineinmyfile:
print(line, end=' ')
finally:
myfile.close() # Optional in Python (auto-closes), but good habit
File Conversion
# Python relies on conversion functions to read from fileslines=open('file.txt').readlines()
lines= [line.rstrip() forlineinlines] # Remove \n# int() ignores \nlin=open('d.txt', 'r')
s=lin.readline() # Returns '89\n'i=int(s) # 89 (ignores \n)
Pickle Module — Object Serialization
importpickle# Save objectD= {'a': 1, 'b': 2}
F=open('database', 'wb') # 'wb' required for picklepickle.dump(D, F)
F.close()
# Load objectF=open('database', 'rb')
E=pickle.load(F) # Automatically converts to dict
ifx<y:
print('less')
elifx>y:
print('greater')
else:
print('equal')
# Ternary expressionresult=value_if_trueifconditionelsevalue_if_falseA=YifxelseZ# Assign Y if x is true, else Z
break# Exit loop immediatelycontinue# Skip to next iterationpass# Do nothing (placeholder)
range() & zip()
range(10) # 0 to 9range(1, 11) # 1 to 10range(0, 20, 2) # 0, 2, 4, ..., 18 (step=2)# zip combines iterableslist(zip([1, 2], ['a', 'b'])) # [(1, 'a'), (2, 'b')]# In Python 3.0+, zip returns iterator; use list() to force
Boolean & Logic
# All objects have boolean value# False: empty objects, 0, None# True: non-empty objects, non-zero, any other value# and/or return actual values (not just True/False)result=XandY# Returns Y if X is True, else Xresult=XorY# Returns X if X is True, else Y# Example: default valuevalue=uservalueordefault_value# Conditional assignmentresult= (XandY) orZ# Y if X is true, else ZX=AorBorCorNone# First non-empty object# in operator for membership'p'inset('spam')
'ham'in ['eggs', 'spam', 'ham']
input() Function
# Get user input from consolereply=input('Enter text: ')
# Example: input loop with validationwhileTrue:
reply=input('Enter text:')
ifreply=='stop':
breakelifnotreply.isdigit():
print('bad'*8)
else:
print(int(reply) **2)
print('Bye')
# Get an iterator from an objectL= [1, 2, 3]
i=iter(L)
i.__next__() # Returns first element (Python 3)i.next() # Python 2# Manual iterationI=iter(L)
whileTrue:
try:
X=next(I)
exceptStopIteration:
breakprint(X**2)
# Single pass iteratorR=range(3)
I1=iter(R)
next(I1) # Works# range() itself is not an iteratorR=range(3)
next(R) # Error! range is iterable, not iterator
Modules & Packages
Import Basics
importmodule# Import modulefrommoduleimportname# Import specific namefrommoduleimport*# Import all (discouraged)importmoduleasm# Aliasfrommoduleimportnameasn# Alias specific
Module Search Path
1. Home directory of program
2. PYTHONPATH directories (if set)
3. Standard library directories
4. .pth file contents (in site-packages)
5. site-packages directory (third-party packages)
# Modules are imported once and cachedimportaa.x=99importa# a is not re-imported, x is still 99# Force reload (Python 3.4+)importimportlibimportlib.reload(a) # Re-execute a's code
Packages
mypackage/
__init__.py # Makes directory a package
module1.py
subpkg/
__init__.py
module2.py
__name__# Current module name (e.g., '__main__' for top-level)__file__# Module's filename__doc__# Module/function docstring__dict__# Namespace dictionary
Main Guard
if__name__=='__main__':
# Code only runs when executed directly, not importedtest_code()
Dynamic Imports
# Reload a module after code changesimportimportlibimportlib.reload(module_name)
# Note: Only works for Python modules, not C extensions
classMyClass:
class_var=0definstance_method(self):
pass@staticmethoddefstatic_method():
return'static'@classmethoddefclass_method(cls):
returncls.class_varMyClass.static_method() # Works without instanceMyClass.class_method() # Receives class, not instance
Slots — Memory Optimization — Python 2.2+
classLimited:
__slots__= ['name', 'age'] # Only these attributes allowedobj=Limited()
obj.name='Alice'# OKobj.job='dev'# AttributeError!
Iteration Support
getitem for indexing
classStepperIndex:
def__getitem__(self, i):
returnself.data[i]
X=StepperIndex()
X.data="spam"foriteminX:
print(item, end=' ') # s p a m
# __contains__ is preferred for "in" operatorclassMyClass:
def__contains__(self, item):
returniteminself.data# __iter__ is preferred for iteration# __getitem__ is fallback for both# Order of preference for "in": __contains__ > __iter__ > __getitem__
Attribute Access Interception
# __getattr__ called when attribute not found elsewhereclassEmpty:
def__getattr__(self, attrname):
ifattrname=='age':
return40else:
raiseAttributeError(attrname)
X=Empty()
X.age# 40X.name# AttributeError: name# __setattr__ called when setting attributes (WATCH FOR INFINITE LOOP!)classControl:
def__setattr__(self, attr, value):
ifattr=='age':
self.__dict__[attr] =value+10# Use __dict__ to avoid recursion!else:
raiseAttributeError(attr+" not allowed")
# __delattr__ is similar to __setattr__ (watch for infinite loop)
Right-side Operations
classadder:
def__add__(self, val):
returnself.val+valdef__radd__(self, val):
returnself.__add__(val)
# Or shorthand__radd__=__add__
# dir() - list all attributesdir(sys) # All attributes in sys module# __doc__ - access docstringclassMyClass:
"""This is my class"""passMyClass.__doc__# "This is my class"# help() - display documentationhelp(MyClass)
Decorators
Function Decorators Basics
# Decorator is a callable that returns a callabledefdecorator(func):
defwrapper(*args, **kwargs):
print('before')
result=func(*args, **kwargs)
print('after')
returnresultreturnwrapper@decoratordefmy_func():
pass# Equivalent to: my_func = decorator(my_func)
Class Decorators
# Using a class to wrap functionsclassdecorator:
def__init__(self, func):
self.func=funcdef__call__(self, *args):
# Do something with self.func and argsreturnself.func(*args)
@decoratordeffunc(arg):
# do somethingpass
classStudent(object):
@propertydefscore(self):
returnself._score@score.setterdefscore(self, value):
ifnotisinstance(value, int):
raiseValueError("score must be an integer")
ifvalue<0orvalue>100:
raiseValueError("score must between 0-100")
self._score=value
importweakref# Weak references don't prevent object reclamation# Useful for caches of large objectsweakref.ref(obj)
Advanced Iteration (itertools)
Module Overview
importitertools# Python 2: many functions return lists# Python 3: most return iterators (using itertools!)
Functions
accumulate(iterable[, func]) — Accumulated sums
# Returns accumulated sums or results of other binary functions# If func supplied, should be function of two arguments# Roughly equivalent to:defaccumulate(iterable, func=operator.add):
it=iter(iterable)
try:
total=next(it)
exceptStopIteration:
returnyieldtotalforelementinit:
total=func(total, element)
yieldtotal# Examplelist(itertools.accumulate([1, 2, 3, 4])) # [1, 3, 6, 10]
*chain(iterables) — Chain multiple iterables
# Returns elements from first iterable until exhausted, then next, etc.# Note: argument is *iterable (multiple args), not single list!itertools.chain('abc', 'def') # a, b, c, d, e, f# WON'T work: itertools.chain(['abc', 'def'])# Use itertools.chain.from_iterable() insteadforiteminitertools.chain([1, 2], [3, 4]):
print(item) # 1, 2, 3, 4
combinations(iterable, r) — r-length combinations
# Return r length subsequences of elements from input iterable# Without replacementitertools.combinations('ABCD', 2) # AB AC AD BC BD CDitertools.combinations([1, 2, 3], 2) # (1,2), (1,3), (2,3)
combinations_with_replacement(iterable, r) — Combinations with replacement
# Return r length subsequences allowing individual elements repeateditertools.combinations_with_replacement('ABC', 2) # AA AB AC BB BC CC
# Return successive r length permutations of elements in iterableitertools.permutations('ABCD', 2) # AB AC AD BA BC BD CA CB CD DA DB DC
compress(data, selectors) — Filter with selector
# Make iterator that filters elements from data # returning only those with corresponding element in selectors that evaluates to True# Stops when either iterable exhausteditertools.compress('ABCDEF', [1, 0, 1, 0, 1, 1]) # A C E F# Roughly equivalent to:defcompress(data, selectors):
return (dford, sinzip(data, selectors) ifs)
count(start=0, step=1) — Infinite counter
# Make iterator that returns evenly spaced values starting with "start"# Often used as argument to map() to generate consecutive data points# Also used in zip() to add sequence numbersitertools.count(10) # 10, 11, 12, 13, ...itertools.count(2.5, 0.5) # 2.5, 3.0, 3.5, ...
cycle(iterable) — Repeat indefinitely
# Make iterator returning elements from iterable and saving a copy# When iterable exhausted, return elements from saved copy# Repeats indefinitelyitertools.cycle('ABCD') # A B C D A B C D ...
dropwhile(predicate, iterable) — Drop until predicate false
# Make iterator that drops elements from iterable as long as predicate is true# Afterwards, returns every element# Note: iterator doesn't produce output until predicate first becomes falseitertools.dropwhile(lambdax: x<5, [1, 4, 6, 4, 1]) # 6, 4, 1
filterfalse(predicate, iterable) — Filter false elements
# Make iterator that filters elements from iterable # returning only those for which predicate is false# If predicate is None, return items that are falseitertools.filterfalse(lambdax: x%2, range(10)) # 0 2 4 6 8
groupby(iterable, key=None) — Group consecutive items
# Make iterator that returns consecutive keys and groups# key is function computing key value for each element# Similar to Unix "uniq"
[kfork, ginitertools.groupby('AAAABBBCCAABBB')] # A B C A B
[[k, list(g)] fork, ginitertools.groupby('AAAABBBCCD')] # [['A', 'AAAA'], ...]# Returns groupby object with (key, group_iterator) tuples
# Returns selected elements from the iterableitertools.islice('ABCDEFG', 2) # A, Bitertools.islice('ABCDEFG', 2, 4) # C, Ditertools.islice('ABCDEFG', 2, None) # C, D, E, F, Gitertools.islice('ABCDEFG', 0, None, 2) # A, C, E, G
*product(iterables, repeat=1) — Cartesian product
# Cartesian product of input iterables# Roughly equivalent to: (x,y) for x in A for y in Bitertools.product('ABCD', 'xy') # Ax Ay Bx By Cx Cy Dx Dyitertools.product(range(2), repeat=3) # 000 001 010 011 100 101 110 111
repeat(object[, times]) — Repeat forever or n times
# Make iterator that returns object over and over again# Repeats indefinitely unless times argument is specifieditertools.repeat('A', 3) # A, A, Aitertools.repeat('A') # A, A, A, A, ... (infinite)
# Make iterator that computes function using arguments obtained from iterable# Used instead of map() when argument parameters already grouped in tuples# Distinction: function(a, b) vs function(*c)itertools.starmap(pow, [(2, 5), (3, 2), (10, 3)]) # 32 9 1000
takewhile(predicate, iterable) — Take until predicate false
# Make iterator that returns elements as long as predicate is true# Once predicate becomes false, we stopitertools.takewhile(lambdax: x<5, [1, 4, 6, 4, 1]) # 1 4
tee(iterable, n=2) — Independent iterators
# Return n independent iterators (in a tuple) from a single iterablec, d=itertools.tee([1, 2, 3], 2)
# Then c and d can iterate through list independently
*zip_longest(iterables, fillvalue=None) — Zip with fill
# Make iterator that aggregates elements from iterables# If iterables are uneven length, missing values filled with fillvalue# Iteration continues until longest iterable exhausteditertools.zip_longest('abcd', 'xy', fillvalue='-') # ax by c- d-
fromcollectionsimportdequed=deque([1, 2, 3])
d.append(4) # Add to rightd.appendleft(0) # Add to leftd.pop() # Remove from rightd.popleft() # Remove from leftd.extend([4, 5, 6]) # Extend rightd.extendleft([...]) # Extend leftd.rotate(n=1) # Rotate n steps rightd.rotate(-1) # Rotate leftd.count(x) # Count occurrencesd.remove(value) # Remove first occurrenced.clear() # Clear alld.reverse() # Reverse in-place# Thread-safe, memory efficient, O(1) operations on both ends
Counter — Tally occurrences
fromcollectionsimportCountercnt=Counter()
forwordin ['red', 'blue', 'red', 'green', 'blue', 'blue']:
cnt[word] +=1cnt# Counter({'blue': 3, 'red': 2, 'green': 1})# Methodscnt.elements() # Iterator over elements repeating eachcnt.most_common([n]) # n most common elements and countscnt.subtract([iterable]) # Subtract countscnt.update([iterable]) # Add counts
OrderedDict — Maintain insertion order
fromcollectionsimportOrderedDictD=OrderedDict([('z', 1), ('a', 2), ('b', 3)])
# Remembers order of element insertionD.popitem(last=True) # Return and remove specified key# last=True: remove last (LIFO), last=False: remove first (FIFO)
defaultdict — Auto-create missing values
fromcollectionsimportdefaultdict# Automatically creates values for missing keysD=defaultdict(list)
D['key'].append(1) # Works even if 'key' doesn't exist# Example: count words grouped by colors= [('yellow', 1), ('blue', 2), ('yellow', 3)]
d=defaultdict(list)
fork, vins:
d[k].append(v)
# d = {'yellow': [1, 3], 'blue': [2]}
Interactive Python (IPython & Jupyter)
IPython — Enhanced Python Interpreter
# Tab completion# Type and press Tab to complete names# Introspection with ?obj? # Display info about objectfunction? # Show docstring and signature
Magic Commands
# Magic commands prefixed with %%timeitstatement# Time execution of statement%timestatement# Report execution time of single statement%debug# Activate interactive debugger%pdb# Inspect stack frames on exception%pwd# Show current working directory%paste# Execute code from clipboard%cpaste# Prompt for code to paste and execute%quickref# Quick reference card%magic# Display all magic commands%hist# Print command input history%reset# Delete all variables%pageOBJECT# Pretty-print object through pager%prunstatement# Profile code with cProfile%who, %whos# Display variables in namespace%xdelvariable# Delete variable and clear references%matplotlib# Configure matplotlib integration%load# Load code from file%runfilename.py# Run Python script in IPython session
Jupyter Notebook
# Browser-based interactive Python
# Similar features to IPython
# Cell-based execution
# Rich output (plots, tables, etc.)
Scientific Computing
SciPy — Scientific Python
fromscipyimportintegrate, linalg, optimize, signal, sparse, special, stats# scipy.integrate - numerical integration, ODE solvers# scipy.linalg - linear algebra (extends numpy.linalg)# scipy.optimize - function optimizers, root finding# scipy.signal - signal processing# scipy.sparse - sparse matrices, sparse linear system solvers# scipy.special - special math functions (SPECFUN Fortran library wrapper)# scipy.stats - probability distributions, statistical tests
# Dictionary is much faster for membership testing# Use dict for fast lookups, not lists# Slowifxin [1, 2, 3, 4, 5, ...]:
pass# Fastifxin {1, 2, 3, 4, 5, ...}:
pass
LEGB Scope Rule
# Local → Enclosing → Global → Built-in# Use global to modify module-level variables# Use nonlocal (Python 3+) to modify enclosing function variables
Comprehension Order (Inner Loop on Right)
# Inner loop is on the RIGHT
[x+yforxin'abc'foryin'lmn']
# Equivalent to:result= []
forxin'abc':
foryin'lmn':
result.append(x+y)
Exception Handling Best Practices
# GOOD: catch specific exceptionstry:
num=int(reply)
exceptValueError:
print('Invalid number')
except: # Only if absolutely necessarypasselse:
print(num**2) # Runs if no exception# Don't use bare except unless absolutely necessary
