What is the purpose of the colon before a block in Python?

The colon is there to declare the start of an indented block.

Technically, it's not necessary; you could just indent and de-indent when the block is done. However, based on the Python koan “explicit is better than implicit” (EIBTI), I believe that Guido deliberately made the colon obligatory, so any statement that should be followed by indented code ends in a colon. (It also allows one-liners if you continue after the colon, but this style is not in wide use.)

It also makes the work of syntax-aware auto-indenting editors easier, which also counted in the decision.

This question turns out to be a Python FAQ, and I found one of its answers by Guido here:

Why are colons required for the if/while/def/class statements?

The colon is required primarily to enhance readability (one of the results of the experimental ABC language). Consider this:

if a == b 
    print a

versus

if a == b: 
    print a

Notice how the second one is slightly easier to read. Notice further how a colon sets off the example in this FAQ answer; it’s a standard usage in English.

Another minor reason is that the colon makes it easier for editors with syntax highlighting; they can look for colons to decide when indentation needs to be increased instead of having to do a more elaborate parsing of the program text.

Three reasons:

1. To increase readability. The colon helps the code flow into the following indented block.
2. To help text editors/IDEs, they can automatically indent the next line if the previous line ended with a colon.
3. To make parsing by python slightly easier.

According to Guido Van Rossum, the Python inventor, the idea of using a colon to make the structure more apparent is inspired by earlier experiments with a Python predecessor, ABC language, which also targeted the beginners. Apparently, on their early tests, beginner learners progressed faster with colon than without it. Read the whole story at Guido's post python history blog.

http://python-history.blogspot.com/2009/02/early-language-design-and-development.html

And yes, the colon is useful in one-liners and is less annoying than the semicolon. Also style guide for long time recommended break on several lines only when it ends with a binary operator

x = (23 + 
     24 + 
     33)

Addition of colon made compound statement look the same way for greater style uniformity.

There is a 'colonless' encoding for CPython as well as colon-less dialect, called cobra. Those did not pick up.

The colon is a complete annoyance. If you need to indent after an 'if' or a 'for', then just look for the 'if' or the 'for'.

Cmon', all this rationalization. The 'spin' language for the propeller chip easily overcomes this issue.

PLEASE, let's make the colon optional, and get on with some logical programming.