Item 7: Consider Conditional Expressions for Simple Inline Logic

Notes

• Python if statements are not expressions
  • The whole block does not evaluate to a result
• Python also supports conditional expressions
  • Provide if/elif/else behaviour anywhere we can use an expression

i = 3
x = "even" if i % 2 == 0 else "odd"
print(x)

odd

• Analogous to a ternary operator in other languages
• In python the result of a truthful expression comes first
• For example, here the fail() is never called, because the middle expression False is the one controlling the branch

def fail():
    raise Exception("Oops")

x = fail() if False else 20
print(x)

20

• if syntax mimics that for filtering
• Here we can use if to control what values are included in a list comprehension

result = [x/4 for x in range(10) if x % 2 == 0]
print(result)

[0.0, 0.5, 1.0, 1.5, 2.0]

• Observe the format is still result precedes the test condition
• Prior to conditional expressions people would emulate them with boolean expressions

i = 3
x = (i % 2 == 0 and "even") or "odd"
print(x)

odd

• This emulates the first conditional expression
  • Relies on understanding python’s quirks for evaluating boolean expressions
  • and returns the first falsely value or last truthy
  • or returns the first truthy or last falsey
• Doesn’t generalise to when you want a falsey result from a truthy condition, e.g.

i = 2
x = (i % 2 == 0 and []) or [1]
print(x)

[1]

• This always evaluates to [1]

• Avoid using boolean expressions

• Can instead use a regular if, else statement

i = 3

if  i % 2 == 0:
    x = "even"
else:
    x = "odd"
print(x)

odd

• This is longer, but
  • Easier to add more logic to each conditional branch
  • e.g. debugging information

i = 3
if i % 2 == 0:
    x = "even"
else:
    x = "odd"
    print("It was odd!")
print(x)

It was odd!
odd

• Can add additional elif blocks

i = 3
if i % 2 == 0:
    x = "even"
elif i % 3 == 0:
    x = "divisible by three"
else:
    x = "odd, and not divisible by three"
print(x)

divisible by three

• If we need the brevity of a conditional expression, we can just write a helper function
  • Which can also be reused

i = 3

def number_group(i):
    if i % 2 == 0:
        return "even"
    else:
        return "odd"
x = number_group(i)
print(x)

odd

• When to then use conditionals?

  • Avoid if they have to split over multiple lines, e.g.

      x = (
          my_long_function_call(1, 2, 3)
          if i % 2 == 0
          else my_other_long_function_call(4, 5, 6)
      )

  • Here the expression is the same length as if we had just written it using a normal if...else construct

• Alternative is assignment expressions
  • Downside is these must be parenthesised in ambiguous contexts
• For example, we could write

x = 2
y = 1
if x and (z := x > y):
    print(z)

True

• But we get a syntax error if we exclude the parenthesis

x = 2
y = 1
if x and z := x > y:
    print(z)

  Cell In[11], line 3
    if x and z := x > y:
       ^
SyntaxError: cannot use assignment expressions with expression

• Not required for conditional expressions
  • Does not mean that it’s any clearer

    if x > y if z else w: # ambiguous
        ...
    if x > (y if z else w): # clear

• Assignment expressions also need parentheses inside a function call argument list

z = dict(your_value=(y := 1))
print(z)

{'your_value': 1}

• Omitting them causes a syntax error

w = dict(other_value = y := 1)
print(w)

  Cell In[13], line 1
    w = dict(other_value = y := 1)
                             ^
SyntaxError: invalid syntax

• Conditional expressions don’t but the longer text may make it harder to read

v = dict(my_value = 1 if x else 3)

Things to Remember

• Conditional expressions let you put an if statement where you might want an expression
• Order of the test expression, true result and false result is different to the typical ternary expression
  • It goes true result if test expression else false result
• Don’t use conditional expressions where they increase ambiguity and reduce readability
• Prefer standard if conditions and helper functions