Nov 04, 2018 ▪ 21 min read (~3 pages) ▪ Computer Science ▪ Updated on Jan 03, 2023
Python program structure (build and run with $ python <filename>.py).
#!/usr/bin/python
def main():
# do something
return
if (__name__ == "__main__"):
main()
Handling exceptions.
try:
# do something
except ZeroDivisionError:
print('Cannot divide with zero.')
# do something
except Exception as e:
print('Error: ' + str(e))
# do something
finally:
# do something
Basic arithmetic operators.
10 + 20
# 30
20 - 10
# 10
10 * 20
# 200
20 / 10
# 2
Power operator.
10 ** 2
# 100
Automatic floating point conversion.
10.0 + (10 + 20)
# 40.0
20.0 - (10 + 10)
# 0.0
10.0 * (10 * 2)
# 200.0
Be aware of this common division error.
30 / 20
# 1
30.0 / 20.0
# 1.5
Integer divison (quotient).
20.0 // 20.0
# 1.0
30.0 // 20.0
# 1.0
40.0 // 20.0
# 2.0
Modulo (remainder).
12.5 % 10.0
# 2.5
10.0 % 20.0
# 10.0
Built in numerical operations.
abs(-20)
# 20
sum([1, 2, 3, 4])
# 10
min(1, 2, 3, 4)
# 1
max(1, 2, 3, 4)
# 4
Rounding values.
round(2.945)
# 3.0
round(2.495)
# 2.0
round(2.945, 2)
# 2.94
Basic variables.
x = 6
y = 'String'
z = 1.05
a = x
x, y, z, a
# (6, 'String', 1.05, 6)
Using type built-in.
type(x)
# int
type(y)
# str
type(z)
# float
type(a)
# int
Multiple assignments.
x = a = 6
y, z = 'String', 1.05
x, y, z, a
# (6, 'String', 1.05, 6)
List assignments.
d, t, v = [230, 45, 12]
# (230, 45, 12)
String assignments.
a, b, c, d = '100B'
# ('1', '0', '0', 'B')
string = '100B'
number = string[:3]
letter = string[3:]
number, letter
# ('100', 'B')
Delete a variable.
del n
n
# NameError: name 'n' is not defined
String indexing.
string = 'proton'
string[:]
# proton
string[:2]
# pr
string[-2:]
# on
string[1:3]
# ro
Reverse a string.
string[::-1]
# notorp
Skip every second character.
string[0:-1:2]
# poo
String operations.
string = 'proton neutron'
string.capitalize()
# Proton neutron
string.title()
# Proton Neutron
string.upper()
# PROTON NEUTRON
string.lower()
# proton neutron
string.center(20, '.')
# ...proton neutron...
string.isdigit()
# False
string.islower()
# True
string.isupper()
# False
string.count('p', 0, len(string))
# 1
string.find('t', 1, len(string))
# 3
string = ' some text '
string.lstrip()
# 'some text '
string.rstrip()
# ' some text'
string.strip()
# 'some text'
Lists are mutable.
list_one = ['REMOVE', 'RANDOM']
list_two = [200, -2, [1.0, 0.0]]
list_one[0] = 'ADD'
# ['ADD', 'RANDOM']
list_one[1]
# RANDOM
Length of lists.
len(list_one)
# 2
len(list_two)
# 3
Concatenate lists.
list = list_one + list_two
# ['ADD', 'RANDOM', 200, -2, [1.0, 0.0]]
List operations.
list.append('NULL')
# ['ADD', 'RANDOM', 200, -2, [1.0, 0.0], 'NULL']
list.sort()
# [-2, 200, [1.0, 0.0], 'ADD', 'NULL', 'RANDOM']
list_string = list('100B')
# ['1', '0', '0', 'B']
Tuples are immutable.
tuple_one = (1.0, 'String', 4)
tuple_two = ('Alpha', 'Bravo', (1, 0))
tuple_one
# (1.0, 'String', 4)
tuple_two
# ('Alpha', 'Bravo', (1, 0))
tuple_two[2][1]
# 0
Concatenate tuples.
tuple = tuple_one + tuple_two
# (1.0, 'String', 4, 'Alpha', 'Bravo', (1, 0))
tuple_list = tuple([100, 'B'])
# (100, 'B')
Dictionaries are key-value pairs.
dict = {
'Adam': ['[email protected]', 2445055],
'Bard': '[email protected]'
}
dict
# {'Adam': ['[email protected]', 2445055], 'Bard': '[email protected]'}
dict['Adam']
# ['[email protected]', 2445055]
dict['Adam'][1]
# 2445055
Dictionaries are mutable.
dict['Bard'] = '[email protected]'
dict
# {'Adam': ['[email protected]', 2445055], 'Bard': '[email protected]'}
Add and remove items.
dict['Cole'] = '[email protected]'
dict
# {'Cole': '[email protected]', 'Adam': ['[email protected]', 2445055], 'Bard': '[email protected]'}
del dict['Cole']
# {'Adam': ['[email protected]', 2445055], 'Bard': '[email protected]'}
'Adam' in dict
# True
dict_list_tuples = dict([(1, "x"), (2, "y"), (3, "z")])
dict_list_tuples
# {1: 'x', 2: 'y', 3: 'z'}
Sets are unordered collections.
set = {1.0, 10, 'String', (1, 0, 1, 0)}
set
# set([(1, 0, 1, 0), 10, 'String', 1.0])
'String' in set
# True
'Java' in set
# False
Add and remove from set.
set.add('Python')
# set(['Python', (1, 0, 1, 0), 10, 'String', 1.0])
set.remove('Python')
# set([(1, 0, 1, 0), 10, 'String', 1.0])
Set operations.
set_one = {1, 2, 3}
# set([1, 2, 3])
set_two = {3, 4, 5}
# set([3, 4, 5])
set_one | set_two
# set([1, 2, 3, 4, 5]) (Union)
set_one & set_two
# set([3]) (Intersection)
set_one - set_two
# set([1, 2]) (Difference)
set_one ^ set_two
# set([1, 2, 4, 5]) (Symmetric difference)
Subset and superset.
set_a = {1, 2}
set_b = {1, 2}
set_c = {1, 2, 3, 4, 5}
set_a < set_b
# False (Strict subset)
set_a <= set_b
# True (Subset)
set_c > set_a
# True (Strict superset)
a = 1.0
b = 5.0
if (a < 1.0):
# do something
elif (a == 1.0):
# do something
else:
# do something
c = (a / b) if a != 0 else a
# 0.2
Booleans.
T = True
F = False
T or F
# True
T and (T and F)
# False
not T
# False
not (not T)
# True
Numbers.
1 == 2
# False
1 != 2
# True
numbers = [1, 2, 3, 4]
for number in numbers:
# do something
Nested for loops.
for i in range(10):
for j in range(10):
# do something
dict = {'Alpha': 1, 'Beta': 2}
for key in dict.keys():
print(key)
# Alpha
# Beta
for value in dict.values():
print(value)
# 1
# 2
for key, value in dict.items():
print(key, value)
# ('Alpha', 1)
# ('Beta', 2)
a = 0
b = 5
while (a < b):
# do something
a += 1
print(a)
# 5
Functions are defined with def keyword.
def function(arg):
# do something
return
Power operator as function.
def power(base, x):
return base ** x
power(2, 3)
# 8
Default arguments.
def power(base, x = 3):
return base ** x
power(2)
# 8
Multiple return values.
def power(base, x):
result = base ** x
return result, base
result, base = power(2, 3)
# (8, 2)
Docstrings.
def function(arg):
'''This is a docstring.'''
return
print (function.__doc__)
# This is a docstring.
Classes are defined with class keyword.
class Money (object):
def __init__(self, amount, currency):
self.amount = amount
self.currency = currency
def __str__(self):
return str(self.amount) + ' ' + self.currency
Create new instance of class.
money = Money(220, 'EUR')
money.amount, money.currency
# (220, 'EUR')
Print use __str__.
print(money)
# 220 EUR
Subclasses.
class VirtualMoney (Money):
def __init__(self, date):
self.date = date
def __str__(self):
return str(self.amount) + ' ' + self.currency + ' (use before ' + self.date + ')'
virtual_money = VirtualMoney('2018-12-31')
virtual_money.amount = 20
virtual_money.currency = 'DIS'
virtual_money
# 20 DIS (use before 2018-12-31)
Reading from files.
with open('path/to/file', 'r') as file:
file.read()
content = open('path/to/file', 'r').read()
Read lines from text-file.
with open('path/to/file.txt', 'r') as file:
for line in file.readlines():
# do something
Write to text-file.
with open('path/to/file.txt', 'w') as file:
file.write('This is some text to write.')
from datetime import datetime
from datetime import timedelta
now = datetime.now()
# 2018-06-15 18:23:51.500993
future = now + timedelta(12)
# 2018-06-27 18:23:59.351647
now.year
# 2018
now.month
# 6
now.day
# 15
now.hour
# 18
now.minute
# 23
Difference between dates.
difference = future - now
difference
# 12 days, 0:00:00
difference.days
# 12
import math
math.pi
# 3.14159265359
math.e
# 2.71828182846
Mathematical operations.
math.floor(2.945)
# 2.0
math.trunc(2.945)
# 2
math.factorial(5)
# 120
math.exp(1)
# 2.71828182846
math.sqrt(16)
# 4.0
math.sin(4 * math.pi / 180)
# 0.0697564737441