Python Conditional (if) Statement Throwing Error in Command Mode One-Liner

I had a situation where I needed to pass code to Python in the form of a single command instead of a script file. While this is easily done by invoking Python in command mode, the code contained a conditional statement that caused Python to throw an error.

Consider this simple Python script named if-test.py. It initialises the variable price, sets its value to 25 and then prints it.

#!/usr/bin/env python3
price=25
print(price)

To run the script we pass the file name to python3 on the command line:

python3 /path/to/if-test.py

25

The same can be achieved by passing the code to python3 not as a script file, but as a command using the -c option (command mode).

python3 -c 'price=25; print(price)'

25

To include a conditional if statement in the script the syntax is:

if condition:
   indented statement block
else:
   indented statement block

Adding an if statement, the script now checks the value of price and if it’s less than 10 prints Cheap otherwise Expensive.

#!/usr/bin/env python3
price=25
if price < 10:
    print("Cheap")
else :
    print("Expensive")

As the value of price is greater than or equal to 10 the script prints Expensive:

python3 /path/to/if-test.py

Expensive

However, passing the same code to python3 in command mode throws a syntax error:

python3 -c 'price=25; if price < 10: print("Cheap") else: print("Expensive")'

  File "<string>", line 1
    price=25; if price < 10: print("Cheap")  else: print("Expensive")
               ^
SyntaxError: invalid syntax

This is because the Python interpreter expects the indented statement blocks for the if statement to be proceeded by a newline and significant leading whitespace as they are in the script file.

I haven’t found a practical way to include newlines nor whitespace when invoking python3 in command mode, but Python – like many other scripting/programming languages – does have a one-line if statement often referred to as a ternary operator. The syntax is:

expression1 if condition else expression2

In our example expression1 is "Cheap", condition is price < 10 and expression2 is "Expensive".

Let’s change the if statement to a ternary operator in the script file:

#!/usr/bin/env python3
price=25
print("Cheap" if price < 10 else "Expensive")

Running the amended script gives:

python3 /path/to/if-test.py

Expensive

As the ternary operator requires no newline nor whitespace it doesn’t throw a syntax error when invoking python3 in command mode:

python3 -c 'price=25; print("Cheap" if price < 10 else "Expensive")'

Expensive

Update 2020-06-03

Here in 2020 I had the same issue using Python in command mode, but this time with a for loop. Unfortunately, there’s no alternative like the ternary operator for the if statement, but I did find a solution for both the for loop and the if statement that uses ANSI-C Quoting.

To recap the original issue, the following command throws an error as the Python interpeter expects the indented statement blocks for the if statement to be proceeded by a newline and significant leading whitespace:

python3 -c 'price=25; if price < 10: print("Cheap") else: print("Expensive")'

  File "<string>", line 1
    price=25; if price < 10 : print("Cheap")  else: print("Expensive")
               ^
SyntaxError: invalid syntax

At the time, I was unable to find a way to insert a newline nor whitespace into the command string passed to the Python interpreter. However, using an ANSI-C-quoted string $'...', backslash escape sequences are treated differently so that \n is interpreted as a newline and \t is interpreted as a tab (whitespace).

The amended ANSI-C-quoted command string when echoed to a terminal window resembles our script file complete with newlines and whitespace (\t can be substituted with 4 spaces):

echo $'price=25\nif price < 10:\n\tprint("Cheap")\nelse:\n\tprint("Expensive")'

price=25
if price < 10:
    print("Cheap")
else:
    print("Expensive")

Passing this ANSI-C-quoted command string to the Python interpreter in command mode no longer throws an error, but gives the expected result:

python3 -c  $'price=25\nif price < 10:\n\tprint("Cheap")\nelse:\n\tprint("Expensive")'

Expensive

It’s worth noting that indented statement blocks don’t always need to be placed on a new line nor actually indented. This code is perfectly valid:

#!/usr/bin/env python3
price=25
if price < 10:print("Cheap")
else:print("Expensive")

Consequently, our ANSI-C-quoted command string can be changed to:

python3 -c  $'price=25\nif price < 10:print("Cheap")\nelse:print("Expensive")'

Expensive

Further examples below show valid scripts using for and while loops. In addition, each example script has been transformed into an ASCI-C-quoted string and passed to the Python interpreter in command mode.

For Loop Examples

#!/usr/bin/env python3
alphabet = ["A", "B", "C", "etc."]
for a in alphabet:
    print(a)

python3 -c $'alphabet = ["A", "B", "C", "etc."]\nfor a in alphabet:\n\tprint(a)'

A
B
C
etc.

#!/usr/bin/env python3
alphabet = ["A", "B", "C", "etc."]
for a in alphabet:print(a)

python3 -c $'alphabet = ["A", "B", "C", "etc."]\nfor a in alphabet:print(a)'

A
B
C
etc.

While Loop Examples

#!/usr/bin/env python3
i = 1
while i < 6:
    print(i)
    i += 1

python3 -c $'i = 1\nwhile i < 6:\n\tprint(i)\n\ti += 1'

1
2
3
4
5

#!/usr/bin/env python3
i = 1
while i < 6:print(i); i += 1

python3 -c $'i = 1\nwhile i < 6:print(i); i += 1'

1
2
3
4
5