3 (Not So) Pythonic Ways to Define a Function in One Line [for Hackers]

Summary: The most Pythonic way to define a function in a single line is to (1) create an anonymous lambda function and (2) assign the function object to a variable name. You can then call the function by name just like any other regularly-defined function. For example, the statement f = lambda x: x+1 creates a function f that increments the argument x by one and returns the result: f(2) returns 3.

Problem: How to define a function in a single line of Python code? This article explores this mission-critical question in all detail!

Example: Say, you want to write the following function in a single line of code:

def f(x):
    return str(x * 3) + '!'

print(f(1))
# 3!

print(f('python'))
# pythonpythonpython!

Let’s get a quick overview of how to accomplish this first:

Exercise: Change the one-liner functions to return the uppercase version of the generated string using the string.upper() function. Then run the code to see if your output is correct!

Method 1: Single-Line Definition

The first and most straightforward way of defining a function in a single line is to just remove the line break:

def f1(x): return str(x * 3) + '!'

print(f1(1))
print(f1('python'))

The function definition is identical to the original one with one difference: you removed the new line and the indentation from the definition. While this works for functions with single-line function bodies, you can easily extend it by using the semicolon as a separator:

>>> def fxx(): x=1; x=2; return x
>>> fxx()
2

Sure, readability gets hurt if you’re doing this but you should still know the syntax in case you see code like this in a practical code project (you will)!

Method 2: Lambda Function

A lambda function is an anonymous function in Python. It starts with the keyword lambda, followed by a comma-separated list of zero or more arguments, followed by the colon and the return expression. For example, lambda x, y, z: x+y+z would calculate the sum of the three argument values x+y+z.

If you need to learn more about the lambda function, check out our ultimate guide on this blog.

Here’s the most Pythonic way to write a function in a single line of code:

f2 = lambda x: str(x * 3) + '!'

print(f2(1))
print(f2('python'))

You create a lambda function and assign the new function object to the variable f2. This variable can now be used like any other function name defined in a regular function definition.

Method 3: exec()

Now, let’s get as unpythonic as we can get, shall we? The exec() function takes one string as an argument. It then executes the code defined in the string argument. In combination with the multi-line character '\n', this enables you to run all complicated multi-line code snippets in a single line. Hackers often use this technique to cram malicious scripts in a seemingly harmless single line of Python code. Powerful, I know.

# Method 3: exec()
f3 = "def f(x):\n    return str(x * 3) + '!'"

exec(f3 + '\nprint(f(1))')
exec(f3 + "\nprint(f('python'))")

The string f3 contains a two-line function definition of our original function f. You then concatenate this string with a new line to print the result of running this newly created function in your script by passing arbitrary arguments into it.

Related Questions

Is it possible to write the if-then-else statement in a single line of code?

Yes, you can write most if statements in a single line of Python using any of the following methods:

  1. Write the if statement without else branch as a Python one-liner: if 42 in range(100): print("42").
  2. If you want to set a variable, use the ternary operator: x = "Alice" if "Jon" in "My name is Jonas" else "Bob".
  3. If you want to conditionally execute a function, still use the ternary operator: print("42") if 42 in range(100) else print("21").

Read more: If-Then-Else in One Line Python [Video + Interactive Code Shell]

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