[Python-ideas] One way to do format and print (was: Desperate need for enhanced print function)

Steven D'Aprano steve at pearwood.info
Mon Sep 7 14:19:35 CEST 2015

On Mon, Sep 07, 2015 at 06:23:13PM +1000, Chris Angelico wrote:

>      print("Hello, I am {b}. My favorite number is {a}.".format_map(locals()))
> This one, though, is a bad idea for several reasons.

Such as what?

> Using locals()
> for formatting is restricted - no globals, no expressions, and no
> nonlocals that aren't captured in some other way. 

That's a feature, not a bug.

locals(), by definition, only includes locals. If you want globals, 
non-locals, built-ins, expressions, or the kitchen sink, you can't get 
them from locals(). Just because the locals() trick doesn't handle every 
possible scenario doesn't make it a bad idea for those cases it does 
handle, any more than it is a bad idea to use 2**5 just because the ** 
operator doesn't handle the 3-argument form of pow().

I probably wouldn't use the locals() form if the variable names were 
hard-coded like that, especially for just two of them:

    "Hello, I am {b}. My favorite number is {a}.".format(a=a, b=b)

Where the locals() trick comes in handy is when your template string is 
not hard-coded:

    if greet:
        template = "Hello, I am {name}, and my favourite %s is {%s}."
        template = "My favourite %s is {%s}."
    if condition:
        template = template % ("number", "x")
        template = template % ("colour", "c")


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