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.

Consider the following list of things to buy from the grocery store, written in Pewprikanese.

pewkah
lalala
    chunkykachoo
    pewpewpew
skunkybacon

When I read that, I'm confused, Are chunkykachoo and pewpewpew a kind of lalala? Or what if chunkykachoo and pewpewpew are indented just because they are special items?

Now see what happens when my Pewprikanese friend add a colon to help me parse the list better: (<-- like this)

pewkah
lalala:   (<-- see this colon)
    chunkykachoo
    pewpewpew
skunkybacon

Now it's clear that chunkykachoo and pewpewpew are a kind of lalala.

Let's say there is a person who's starting to learn Python, which happens to be her first programming language to learn. Without colons, there's a considerable probability that she's going to keep thinking "this lines are indented because this lines are like special items.", and it could take a while to realize that that's not the best way to think about indentation.

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.

As far as I know, it's an intentional design to make it more obvious, that the reader should expect an indentation after the colon.

It also makes constructs like this possible:

if expression: action()
code_continues()

Note (as a commenter did) that this is not exactly the shining gold standard of good Python style. It would be far better to have a blank, there:

if expression: action()

code_continues()

to avoid confusion. I just wanted to make it clear, with the first example, that it's possible to write like that, since having the code for the if immediately following the colon makes it possible for the compiler to understand that the next line should not be indented.