Python Cheat Sheet

python-cheatsheet.md

P53 Python Cheatsheet

Expressions

Basic Types

• bool: yes = True, no = False
• int: year = 2001
• float: g = 9.81, G = 6.67e-11
• str: name = "Welcome to 'P53'"

Collection Types

• list: vec = [ 1.23, -0.5, 2 ]
• tuple: date = (1999, 12, 31)

Arithmetic Operators

• 1 + 1 = 2, -1 - 1 = -2
• "hi" + "world" = "hiworld"
• 2 * 3 = 6, 3 / -2 = -1.5
• "hi" * 3 = "hihihi"
• power: 2 ** 3 = 8
• modulus: 7 % 3 = 1
• floor div: 7 // 3 = 2

Comparison Operators

• equality: 1 + 1 == 2 is True
• inequality: 1 + 1 != 2 is False
• 2 >= 1 is True, 2 > 2 is False
• 2 <= 1 is False, 2 < 3 is True

Logical Operators

• True and False is False
• True or False is True
• not True is False
• 2 >= 1 or 2 > 2 is True

Indexing

• now = ['Apr',13,[9,15,'am']]
• now[0] is 'Apr'
• now[1] and now[-2] are 13
• now[2] is [9,15,'am']
• now[2][-1] is 'am'

Slicing

• digits=[0,1,2,3,4,5,6,7,8,9]
• digits[:4] is [0,1,2,3]
• digits[6:] is [6,7,8,9]
• digits[4:6] is [4,5]
• digits[2:8:2] is [2,4,6]
• len(digits[2:3]) is 1

List Comprehension

• [2*n+1 for n in range(5)]
• [n-1 for n in range(5) if n%2==1]

Tuples

• can be indexed and sliced like lists
• cannot be modifed after creation
• tuple(1, 2, 3)
• tuple([1, 2, 3])
• (1, 2, 3)
• 1, 2, 3
• (1,) has length 1
• (,) has length 0

String Interpolation

a = 1
b = 3
print(f'a={a}, b={b}, a/b={a/b:.2f}')

Control Flow

if ?:
    pass
elif ?:
    pass
else:
    pass

while ?:
    pass

for value in values:
    pass

for i, value in enumerate(values):
    pass

• break: break out of enclosing loop
• continue: jump to next iteration

Functions

Built-In

• utility: print,help
• numeric: abs,min,max,pow,round
• types: bool,int,float,str,list,tuple
• iterable: len,enumerate,range

Math Library

• import math
• math.sin(math.pi / 2) is 1.0

User-Defined Functions

def myfunc(x, y):
    return x + y, x - y

assert myfunc(1, 2) == (3, -1)

def combine(x1, x2, c1=1, c2=1):
    return c1 * x1 + c2 * x2

assert combine(2, 3) == 5
assert combine(2, 3, c1=0.5) == 4
assert combine(2, 3, c2=2) == 8

Numerical Python

• import numpy as np

Array Creation

• x = np.array([0.0,0.2,0.4,0.8,1.0])
• x = np.linspace(0, 1, 5)
• np.zeros((2, 3)) is 2x3 array of zeros
• np.ones((3, 2, 3)) is 3x2x3 array of ones

Array Attributes

• x.shape is tuple of dimensions
• x.dtype is type of each element value
• x.size is total number of elements

Math

• trig: sin,cos,tan,asin,acos,atan,atan2
• angles: deg2rad,rad2deg
• constants: pi,e,inf,nan

Random Numbers

• rng = np.random.default_rng(seed=123)
• rng.uniform(lo, hi, size=(3, 2, 3))
• rng.normal(loc=mu, scale=stddev, size=9)

Plotting

• import matplotlib.pyplot as plt
• plt.plot(x, y, 'r-')
• plt.hist(z, bins=50, range=(-5,5))
• colors: r(ed),g(reen),b(lue),(blac)k
• line styles: - -- :
• symbols: . o + x