If a b or c python

This question already had many highly upvoted answers and an accepted answer, but all of them so far were distracted by various ways to express the boolean problem and missed a crucial point:

I have a python script that can receive either zero or three command line arguments. (Either it runs on default behavior or needs all three values specified)

This logic should not be the responsibility of library code in the first place, rather it should be handled by the command-line parsing (usually argparse module in Python). Don't bother writing a complex if statement, instead prefer to setup your argument parser something like this:

#!/usr/bin/env python
import argparse

parser = argparse.ArgumentParser()
parser.add_argument('--foo', nargs=3, default=['x', 'y', 'z'])
args = parser.parse_args()

print(args.foo)

And yes, it should be an option not a positional argument, because it is after all optional.

edited: To address the concern of LarsH in the comments, below is an example of how you could write it if you were certain you wanted the interface with either 3 or 0 positional args. I am of the opinion that the previous interface is better style (because optional arguments should be options), but here's an alternative approach for the sake of completeness. Note we're overriding kwarg usage when creating your parser, because argparse will auto-generate a misleading usage message otherwise!

#!/usr/bin/env python
import argparse

parser = argparse.ArgumentParser(usage='%(prog)s [-h] [a b c]\n')
parser.add_argument('abc', nargs='*', help='specify 3 or 0 items', default=['x', 'y', 'z'])
args = parser.parse_args()
if len(args.abc) != 3:
    parser.error('expected 3 arguments')

print(args.abc)

Here are some usage examples:

# default case
$ ./three_or_none.py 
['x', 'y', 'z']

# explicit case
$ ./three_or_none.py 1 2 3
['1', '2', '3']

# example failure mode
$ ./three_or_none.py 1 2 
usage: three_or_none.py [-h] [a b c]
three_or_none.py: error: expected 3 arguments

Python If ... Else

Python Conditions and If statements

Python supports the usual logical conditions from mathematics:

• Equals: a == b
• Not Equals: a != b
• Less than: a < b
• Less than or equal to: a <= b
• Greater than: a > b
• Greater than or equal to: a >= b

These conditions can be used in several ways, most commonly in "if statements" and loops.

An "if statement" is written by using the if keyword.

Example

If statement:

a = 33
b = 200
if b > a:
  print("b is greater than a")

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In this example we use two variables, a and b, which are used as part of the if statement to test whether b is greater than a. As a is 33, and b is 200, we know that 200 is greater than 33, and so we print to screen that "b is greater than a".

Indentation

Python relies on indentation (whitespace at the beginning of a line) to define scope in the code. Other programming languages often use curly-brackets for this purpose.

Example

If statement, without indentation (will raise an error):

a = 33
b = 200
if b > a:
print("b is greater than a") # you will get an error

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Elif

The elif keyword is pythons way of saying "if the previous conditions were not true, then try this condition".

Example

a = 33
b = 33
if b > a:
  print("b is greater than a")
elif a == b:
  print("a and b are equal")

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In this example a is equal to b, so the first condition is not true, but the elif condition is true, so we print to screen that "a and b are equal".

Else

The else keyword catches anything which isn't caught by the preceding conditions.

Example

a = 200
b = 33
if b > a:
  print("b is greater than a")
elif a == b:
  print("a and b are equal")
else:
  print("a is greater than b")

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In this example a is greater than b, so the first condition is not true, also the elif condition is not true, so we go to the else condition and print to screen that "a is greater than b".

You can also have an else without the elif:

Example

a = 200
b = 33
if b > a:
  print("b is greater than a")
else:
  print("b is not greater than a")

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Short Hand If

If you have only one statement to execute, you can put it on the same line as the if statement.

Example

One line if statement:

if a > b: print("a is greater than b")

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Short Hand If ... Else

If you have only one statement to execute, one for if, and one for else, you can put it all on the same line:

Example

One line if else statement:

a = 2
b = 330
print("A") if a > b else print("B")

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This technique is known as Ternary Operators, or Conditional Expressions.

You can also have multiple else statements on the same line:

Example

One line if else statement, with 3 conditions:

a = 330
b = 330
print("A") if a > b else print("=") if a == b else print("B")

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And

The and keyword is a logical operator, and is used to combine conditional statements:

Example

Test if a is greater than b, AND if c is greater than a:

a = 200
b = 33
c = 500
if a > b and c > a:
  print("Both conditions are True")

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Or

The or keyword is a logical operator, and is used to combine conditional statements:

Example

Test if a is greater than b, OR if a is greater than c:

a = 200
b = 33
c = 500
if a > b or a > c:
  print("At least one of the conditions is True")

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Nested If

You can have if statements inside if statements, this is called nested if statements.

Example

x = 41

if x > 10:
  print("Above ten,")
  if x > 20:
    print("and also above 20!")
  else:
    print("but not above 20.")

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The pass Statement

if statements cannot be empty, but if you for some reason have an if statement with no content, put in the pass statement to avoid getting an error.

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