Introduction to Python

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Object Methods

object methods

Objects are capable of behaviors, which are expressed as methods.

var = 'Hello, World!'

var2 = var.replace('World', 'Mars')      # replace substring, return a str

print(var2)                              # Hello, Mars!

• Methods are type-specific functions that are used only with a particular type. They work with the object itself, and any values passed as arguments.
• We commonly say "call the x method on this object".
• Methods use the object itself as its implied first value. Any action the method takes will make use of, or modify, the object on which it is called.

methods vs. functions

Compare method syntax to function syntax.

mystr = 'HELLO'

x = len(mystr)            # function len() (stands alone)

y = mystr.count('L')      # method .count() (attached to the string variable)

Methods and functions are both called (using the parentheses after the name of the function or method). Both also may take an argument and/or may return a return value.

string method: .upper()

This "transforming" method returns a new string with a string's value uppercased.

var = 'hello'
newvar = var.upper()        # str, 'HELLO'

print(newvar)               # 'HELLO'

This method does not take an explicit argument, because it works with the string object itself.

string method: .lower()

This "transforming" method returns a new string with a string's value lowercased.

var = 'Hello There'
newvar = var.lower()        # str, 'hello there'

print(newvar)               # 'hello there'

This method does not take an explicit argument, because it works with the string object itself.

string method: .replace()

This "transforming" method returns a new string based on an old string, with specified text replaced.

var = 'My name is Marie'

newvar = var.replace('Marie', 'Greta')    # str, 'My name is Greta'

print(newvar)                             # My name is Greta

This method takes two arguments, the search string and replace string.

string method: .isdigit()

This "inspector" method returns True if a string is all digits.

mystring = '12345'
if mystring.isdigit():
    print("that string is all numeric characters")

if not mystring.isdigit():
    print("that string is not all numeric characters")

Since it returns True or False, inspector methods like isdigit() are used in an if or while expression. To test the reverse (i.e. "not all digits"), use if not before the method call.

string method: .endswith()

This "inspector" method returns True if a string starts with or ends with a substring.

bb = 'This is a sentence.'
if bb.endswith('.'):
    print("that line had a period at the end")

string method: .startswith()

This "inspector"method returns True if the string starts with a substring.

cc = input('yes? ')
if cc.startswith('y') or cc.startswith('Y'):
    print('thanks!')
else:
    print("ok, I guess not.")

string method: .count()

This "inspector" method returns a count of occurrences of a substring within a string.

aa = 'count the substring within this string'
bb = aa.count('in')
print(bb)             # 3 (the number of times 'in' appears in the string)

string method: .find()

This "inspector" method returns the character position of a substring within a string.

xx = 'find the name in this string'
yy = xx.find('name')
print(yy)             # 9 -- the 10th character in mystring

ex. 3.27 - 3.28

f'' strings for string formatting

An f'' string allows us to embed any value (such as numbers) into a new, completed string.

aa = 'Jose'
var = 34

bb = f'{aa} is {var} years old.'

print(bb)                                  # Jose is 34 years old.

• f'' strings are the preferred way to combine strings with numbers
• you do not need to convert or concatenate
• it's also recommended not to use commas, i.e. print(aa, 'is', var); this is best use for diagnostic purposes, but not final output

Ex. 3.29

f'' string format codes

There are numerous options for justifying, formatting numbers, and more.

overview of formatting

# text padding and justification
# :<15     # left justify width
# :>10     # right justify width
# :^8      # center justify width

# numeric formatting
:f         # as float (6 decimal places)
:.2f       # as float (2 decimal places)
:,         # 000's comma separators
:,.2f      # 000's comma separators with float rounded to 2 places

f'' string format code examples

There are even more options, you can search online for details.

examples

x = 34563.999999

f'hi:  {x:<30}'      # 'hi:  34563.999999                  '

f'hi:  {x:>30}'      # 'hi:                    34563.999999'

f'hi:  {x:^30}'      # 'hi:           34563.999999         '

f'hi:  {x:f}'        # 'hi:  34563.999999'

f'hi:  {x:.2f}'      # 'hi:  34564.00'

f'hi:  {x:,}'        # 'hi:  34,563.999999'

f'hi:  {x:,.2f}'     # 'hi:  34,564.00'

Please note that f'' strings are available only as of Python 3.6. ex 3.29

sidebar: method and function return values in an expression; combining expressions

The return value of an expression can be used in another expression.

letters = "aabbcdefgafbdchabacc"

vara = letters.count("a")         # 5

varb = len(letters)               # 20

varc = vara / varb                # 5 / 20, or 0.25

vard = varc * 100                 # 25


print(len(letters) / letters.count("a") * 100)  # statements combined

• the first 4 statements calculate the percentage of a's in the string
• the last statement does these same operations in one statement
• combining statements is optional, but can be fun
• shorter code is usually better, as long as it is clear

a note on style in your homework submissions

Professional coders respect good style because it makes code easier to read.

• good and proper style is important within the developer community
• proper style is easier to read
• it is also the mark of professionalism
• code with bad style just looks wrong to an experienced developer
• therefore if you don't employ good style, your code appears amateurish
• a style guide is included in your supplementary materials
• the guide covers the bare basics of well styled code
• if you get into the right habit now, you'll always be creating profesionally-styled code
• if you are interested in even more style conventions, check out the document online called PEP 8 (Python style guide)