Python: how do I call `print` from `eval` in a loop?

Question

When I call print from eval:

def printList(myList):
    maxDigits = len(str(len(myList)))
    Format = '0{0}d'.format(maxDigits)
    for i in myList:
        eval('print "#{0:' + Format + '}".format(i+1), myList[i]')

it gives an error:

    print "#{0:01d}".format(i+1), myList[i]
        ^
SyntaxError: invalid syntax

I tried to make use of this, and re-wrote it:

def printList(myList):
    maxDigits = len(str(len(myList)))
    Format = '0{0}d'.format(maxDigits)
    for i in myList:
        obj = compile(src, '', 'exec')
        eval('print "#{0:' + Format + '}".format(i+1), myList[i]')

but this complains about the i:

NameError: name 'i' is not defined

P.S. I'm dealing with python2.6

Which version of python? Unless I'm mistaken, print syntax is different in python 3. — Marcin, Feb 17 '12 at 14:42
@Gerrat: I'm using `eval` because I want to substitute the format specification. See the `Format = ` string. — Adobe, Feb 17 '12 at 14:45
@Adobe: This isn't a great use for eval (there aren't many). You can just substitute `Format` this into your string to begin with. No need to eval it. — Gerrat, Feb 17 '12 at 14:50

score 16 · Answer 1 · edited May 23 '17 at 10:28

You can't eval() a print: eval() is used to evaluate expression, and print is a statement. If you want to execute a statement, use exec(). Check this question for a better explanation:

>>> exec('print "hello world"')
hello world

Now, you can pass your locals() variables if you want to make accessible the i in the exec:

>>> i = 1
>>> exec('print "hello world", i', locals())
hello world 1

In addition, in the last test you wrote, you compile() in 'exec' mode, that should give you a tip :)

eumiro · Accepted Answer · 2012-02-17T18:21:32.527

8

You don't need eval:

def printList(myList):
    maxDigits = len(str(len(myList)))
    str_format = '#{0:0' + str(maxDigits) + '}'
    for i, elem in enumerate(myList, 1):
        print str_format.format(i), elem

or, as @SvenMarnach noted, you can put even the formatting parameter into one format call:

def printList(myList):
    maxDigits = len(str(len(myList)))
    for i, elem in enumerate(myList, 1):
        print '#{1:0{0}} {2}'.format(maxDigits, i, elem)

edited Feb 17 '12 at 18:21

answered Feb 17 '12 at 14:51

eumiro

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4

You don't even need to build `str_format`. `print "{0:0{1}}".format(i, maxDigits)` will just work fine. – Sven Marnach Feb 17 '12 at 15:15
Funny that your comment is being voted for better than my answer, although I posted it first :p – Gandaro Feb 17 '12 at 18:09
@Gandaro - and he gets one more from me (you too), because this nesting was new to me. – eumiro Feb 17 '12 at 18:22

score 3 · Answer 3 · answered Feb 17 '12 at 15:06

To keep your code while making it shorter and easier to understand:

def printList(myList):
    # int(math.log10(len(myList))+1) would be the appropriate way to do that:
    maxDigits = len(str(len(myList)))
    for i in myList:
        print "#{0:0{1}d}".format(i+1, maxDigits), myList[i]

S.Lott · Answer 4 · 2012-02-17T15:16:19.657

1

The simplistic view is this. Build the format separately from using it. Avoid eval().

    format =  "#{0:" + Format + "}"
    print format.format(i+1), myList[i]

Don't make things harder than they need to be. Here's another version that builds the format in one step.

    format = '#{{0:0{0}d}}'.format(maxDigits)
    print format.format(i+1), myList[i]

edited Feb 17 '12 at 15:16

answered Feb 17 '12 at 14:52

S.Lott

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ValueError: Invalid conversion specification – Gandaro Feb 17 '12 at 14:58

Python: how do I call `print` from `eval` in a loop?

4 Answers4