Basic Python Usage Notes

Basic Python Notes.py

# -----------------------------------------------
# Loops
# -----------------------------------------------
aList = ['apple', 'orange', 'pear']

## print all fruits
for fruit in aList:
    print(fruit)

## skip 'apple'
for fruit in aList:
    if fruit == 'apple':
        continue
    print(fruit)

## stop at 'orange'
for fruit in aList:
    if fruit == 'orange':
        break
    print(fruits)

# -----------------------------------------------
# range
# -----------------------------------------------
## looping through a range of numbers
for i in range(...):
    print(i)

range(10)           # 0 to 10, excluding 10 -> 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
range(2,6)          # 2 to 6, excluding 6 -> 2, 3, 4, 5
range(2, 20, 3)     # 2 to 30, excluding 20, step 3 -> 2, 5, 8, 11, 14, 17
range(5, 1, -1)     # 5 to 1, excluding 1, backwards -> 5, 4, 3, 2

for i in range(len(aList)):
    print(i, aList[i])

## better way to loop a list to get both the index and value
for i, fruit in enumerate(aList): # i - index, fruit - value
    print(i, fruit)


# -----------------------------------------------
# Reversing a list
# -----------------------------------------------
## loop a list backwards
for fruit in reversed(aList):
    print(fruit)

for i, fruit in enumerate(reversed(aList)):
    print(i, fruit)

## To get a copy of reversed list
bList = list(reversed(aList)) # ['pear', 'orange', 'apple'], aList unchanged

## Reverse aList 'in place'
aList.reverse() # aList is now ['pear', 'orange', 'apple']

# -----------------------------------------------
# Indexing a string
# -----------------------------------------------
aStr = 'abcdefghijklmn'

aStr[1] == 'b'          # char at position 1
aStr[2:5] == 'cde'      # characters from position 2 to 4
aStr[::2] == 'acegikm'  # every second character
aStr[:3] == 'abc'       # from 0 to 2
aStr[5:] == 'fghijklmn' # starting from 5
aStr[::-1] == 'nmlkjihgfedcba' # reverse
aStr[2::-1] == 'cba'    # starting from 2, backwards

# slice(,,) function works the same way
aStr[slice(2, 5)] == aStr[2:5] # why? you can assign slice to a variable and reuse


# -----------------------------------------------
# if statements
# -----------------------------------------------
a = 20
b = 30

if a > b:
    print('a greater than b')
else:
    print('b not greater than a')

if a == b:
    print('a equals to b')
elif a > b:
    print('a greater than b')
else:
    print('a not equals to b and a not greater than b')

# -----------------------------------------------
# finding things in a loop
# -----------------------------------------------
foundApple = False
for fruit in aList:
    if fruit == 'apple':
        foundApple = True
        break
print(foundApple)

# find position of first 'apple' in a list
applePosition = -1
for i, fruit in enumerate(aList):
    if fruit == 'apple':
        applePosition = i
        break
print(applePosition)

# find position of last 'apple' in a list
applePosition = -1
for i in range(len(aList)-1, -1, -1):
    if aList[i] == 'apple':
        applePosition = i
        break
print(applePosition)

## alternative
applePosition = -1
for i, fruit in enumerate(aList):
    if fruit == 'apple':
        applePosition = i   # notice: no break, this is inefficient because it has to go through the entire list
print(applePosition)

## alternative
applePosition = -1
for i, fruit in enumerate(reversed(aList)):
    if fruit == 'apple':
        applePosition = len(aList) - 1 - i  # starting at len(aList)-1
        break
print(applePosition)

# -----------------------------------------------
# find maximum
# -----------------------------------------------
numList = [10, 30, 12, 34, 70, 3]
maxValue = max(numList)

## or with a for loop
maxValue = numList[0]   # use the first value as the potential maximum
for i in numList[1:]:   # loop from the next value (but start from 0 will work as well)
    if i > maxValue:
        maxValue = i
print(i)

# -----------------------------------------------
# sorted
# -----------------------------------------------
clothes = ['shirt', 'hat', 'socks', 'shorts'] 
for cloth in sorted(clothes):
  print(cloth) # hat, shirt, shorts, socks

for cloth in sorted(clothes, key=len):  # Sort by length
  print(cloth) # hat, shirt, socks, shorts

for cloth in sorted(clothes, key=str.casefold):  # ignoring upper/lower case
  print(cloth) # hat, shirt, shorts, socks


# -----------------------------------------------
# Dictionaries
# -----------------------------------------------
scores = { 'david': 85, 'peter': 70, 'andrew': 69, 'alexander': 91 }
print(scores['alexander'])

for name, score in scores.items(): # get the items in a dictionary as a list
    print(name, score)

# Nested dictionaries
classScores = {
    'class1': { 'david': 85, 'peter': 70 },
    'class2': { 'andrew': 69, 'alexander': 91 }
}

for className, students in classScores.items():
    print('--------')
    print(className, len(students))
    for name, score in students.items():
        print(name, score)



# -----------------------------------------------
# Parsing string in python
# -----------------------------------------------
## split string into a list, using " " as separator
myString = "What does the fox say?"
print(myString.split(" "))  
    # ["What", "does", "the", "fox", "say?"]

## split 2 separators only
print(myString.split(" ", 2))  
    # ["What", "does", "the fox say?"]

## split into a tuple: before and after the partition string
print(myString.partition("the")) 
    # ("What does ", "the", " fox say?")

## removes starting and ending spaces
stringWithSpaces = "     some string    "
print(stringWithSpaces.strip()) 
    # "some string"

## strip left side
print(stringWithSpaces.lstrip()) 
    # "some string    "

print(stringWithSpaces.rstrip()) # strip left side
    # "    some string"

stringWithUpperAndLower = "aBcDeFg xYz"
print(stringWithUpperAndLower.upper())
    # "ABCDEFG XYZ"

print(stringWithUpperAndLower.lower())
    # "abcdefg xyz"

print(myString.startsWith("What")) # true
print(myString.startsWith("When")) # false
print(myString.endsWith("?")) # true
print("fox" in myString) # true

# -----------------------------------------------
# Printing with formatting
# -----------------------------------------------
className = 'COMP-204'
assignmentName = 'print something'
print("Class: {}, Assignment: {}".format(className, assignmentName))
print(f"Class: {className}, Assignment: {assignmentName}") 
    # see the f prefix to the string is called an interpolated string,
    # you can put python code betwen the {} to print the expression's value
    
    
# -----------------------------------------------
# Reading from file
# -----------------------------------------------
f = open("somefile.txt", "r")
for line in f:
    print(line)
    
# loading files into a nested dictionary
# E.g. Files containing lines with
# className,studentName,score

f = open("somefile.txt", "r")
classes = {} # Initialize classes as a dictionary
for line in f:
    className, studentName, score = line.split(',', 2)
    if not className in classes:    # check to see if the className is already in the classes dictionary
        classes[className] = {}     # if not, initialize it with an empty dictionary
    classes[className][studentName] = score # set the studentName/score in the class's dictionary
print(classes)