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
Answers:
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)
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 🙂
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]
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]
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
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)
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 🙂
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]
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]