Python from zero: part 3: functions and reading errors

1 · Observe

You have already used functions

Words used in this lesson
Word	What it means
Function	A named recipe that takes inputs and gives back an answer. print and len are functions.
Argument	A value you give a function when you use it: in len(prices), prices is the argument.
Return	The answer a function hands back to the line that used it.
Method	A function that belongs to a value, written with a dot: "pune".upper().
Error (exception)	Python stopping because it could not do what a line asked. It prints the kind and a message.
Traceback	The report Python prints with an error: where it stopped and why.

Every time you wrote print(...) or len(...) you used a function: a recipe someone wrote once so everyone could reuse it. You can write your own. If you find yourself copying the same three lines with different numbers, that is a function waiting to be named.

2 · Express

def, return and the dot

def name(input1, input2):

steps using the inputs

return the_answer

def starts the recipe, the indented lines are the steps, and return hands back the answer.

Using the function is called calling it: write its name, then the inputs in brackets. The inputs you pass in are matched to the names in the def line, in order.

The functions you will use constantly
Function	What it does	Example	Gives
print(x)	shows x	print(5)	shows 5
len(x)	how many items	len([4, 8, 9])	3
type(x)	what kind of value	type(2.5)	float
round(x, n)	round to n decimals	round(3.14159, 2)	3.14
sum(list)	adds the values	sum([1, 2, 3])	6
min(list), max(list)	smallest, largest	max([4, 8, 9])	9
sorted(list)	a sorted copy	sorted([3, 1, 2])	[1, 2, 3]
int(x), float(x), str(x)	convert type	int("42")	42
abs(x)	distance from zero	abs(-7)	7

A method is a function that belongs to a value. You write the value, a dot, the method, and brackets: "pune".upper() gives "PUNE", and prices.append(80.0) adds a value to a list.

Check yourself

What is the difference between print(x) and return x?

None, they do the same thing print shows something; return hands the value back so you can use it return shows it on the screen

3 · Visualize

What happens when you call a function

Tracing price_per_sqft(62.5, 1075)
Step	What happens	Names inside the function
1	The inputs are matched in order to the names in the def line	price_lakh → 62.5, area_sqft → 1075
2	rupees = price_lakh * 100000	rupees → 6250000.0
3	return rupees / area_sqft	Answer 5813.95… is handed back
4	The line that called the function receives it	rate → 5813.95…

4 · Discover

Write one and call it

def: give a recipe a name

def price_per_sqft(price_lakh, area_sqft):     # the recipe: two inputs
    rupees = price_lakh * 100000
    return rupees / area_sqft                  # the answer it hands back

print(price_per_sqft(62.5, 1075))              # use the recipe
print(price_per_sqft(48.0, 900))               # use it again with new inputs
rate = price_per_sqft(55.5, 1000)              # store what it hands back
print(round(rate, 1))

5813.953488372093
5333.333333333333
5550.0

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def: give a recipe a name

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The first call prints 5813.953488372093, because Python shows a float with all its digits. Use round() when you only want a few. Now the difference between showing and handing back.

print shows, return hands back

def shows_it(x):
    print("showing:", x * 2)

def hands_it_back(x):
    return x * 2

a = shows_it(5)           # prints, but gives back nothing useful
b = hands_it_back(5)      # gives back 10
print("a is", a)
print("b is", b)
print("b plus 1 is", b + 1)

showing: 10
a is None
b is 10
b plus 1 is 11

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print shows, return hands back

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Python code (editable)

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The function that only printed gave back nothing useful: a is None, which is Python's word for 'no value'. The function with return gave back 10, which we could add to. In data work you almost always want return.

Functions that come with Python

prices = [62.5, 48.0, 71.2, 55.5]
print(len(prices), min(prices), max(prices), sum(prices))
print(sorted(prices))
print(round(3.14159, 2), abs(-7))
print(int("42") + 1, float("2.5") * 2, str(100) + " lakh")
print("pune".upper(), "  Pune ".strip(), "a,b,c".split(","))

4 48.0 71.2 237.2
[48.0, 55.5, 62.5, 71.2]
3.14 7
43 5.0 100 lakh
PUNE Pune ['a', 'b', 'c']

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Functions that come with Python

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5 · Apply

Reuse the recipe, then read the errors

Example 1: use the same function on a whole list of flats. Each item is a pair, and a for loop can unpack the pair into two names.

A function used inside a loop

def price_per_sqft(price_lakh, area_sqft):
    return price_lakh * 100000 / area_sqft

flats = [(62.5, 1075), (48.0, 900), (55.5, 1000)]    # (price in lakh, area in sq ft)
for price, area in flats:
    print(price, "lakh for", area, "sq ft =", round(price_per_sqft(price, area)), "a sq ft")

62.5 lakh for 1075 sq ft = 5814 a sq ft
48.0 lakh for 900 sq ft = 5333 a sq ft
55.5 lakh for 1000 sq ft = 5550 a sq ft

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A function used inside a loop

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Python code (editable)

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Example 2 is the habit that matters most for beginners: when Python stops, read the last line of the error. It has two parts, the kind of error and a message. Here are the six you will meet most, each caused on purpose.

Each error has a name and a message

def show(label, fn):
    try:
        fn()
    except Exception as e:
        print(f"{label}: {type(e).__name__}: {e}")

prices = [62.5, 48.0]
show("missing name ", lambda: print(pirce))
show("wrong type   ", lambda: "5" + 5)
show("bad position ", lambda: prices[5])
show("missing key  ", lambda: {"city": "Pune"}["state"])
show("divide by 0  ", lambda: 1 / 0)
show("bad number   ", lambda: int("twelve"))

missing name : NameError: name 'pirce' is not defined
wrong type   : TypeError: can only concatenate str (not "int") to str
bad position : IndexError: list index out of range
missing key  : KeyError: 'state'
divide by 0  : ZeroDivisionError: division by zero
bad number   : ValueError: invalid literal for int() with base 10: 'twelve'

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Each error has a name and a message

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Python code (editable)

Press Ctrl+Enter (⌘+Enter on Mac) to run. Tab moves focus to the next control, as usual; use spaces to indent.

What each error usually means
Error	Usually means	First thing to check
SyntaxError	The line is not valid Python	Missing colon, bracket or quote; = where == was meant
NameError	A name that does not exist yet	Spelling, capital letters, quotes missing around text, run order
TypeError	Wrong kind of value for the job	Mixing text and numbers; wrong number of inputs
IndexError	A list position that does not exist	Counting from 0; the list is shorter than you think
KeyError	A dictionary key that is not there	Spelling of the key
ZeroDivisionError	Dividing by zero	The divisor; an empty list
ValueError	Right type, wrong content	int("twelve") cannot become a number

6 · Where it goes wrong

Habits that save hours

When stuck, print the value you are unsure about. print(type(x), x) is the quickest way to see what Python actually holds.

7 · In Python

Everything together

This program has a function, a decision inside it, a loop and an f-string: every idea from the three parts.

Parts 1 to 3 together

def price_per_sqft(price_lakh, area_sqft):
    return price_lakh * 100000 / area_sqft

def verdict(rate, limit):
    if rate <= limit:
        return "good value"
    return "expensive"

flats = [("Baner", 62.5, 1075), ("Hadapsar", 48.0, 900), ("Kothrud", 71.2, 1000)]
for name, price, area in flats:
    rate = price_per_sqft(price, area)
    print(f"{name}: {round(rate)} a sq ft, {verdict(rate, 6000)}")

Baner: 5814 a sq ft, good value
Hadapsar: 5333 a sq ft, good value
Kothrud: 7120 a sq ft, expensive

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Parts 1 to 3 together

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Python code (editable)

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Change the limit from 6000 to 5000. Which flats change verdict?
Add a fourth flat to the list and run it again.
Break it on purpose: delete the colon after def verdict(rate, limit) and read the error. Then put it back.