[SOLVED] Python: Create variables from dictionary keys

danubuntuman · 01-31-2011, 09:12 AM

Hi all,

I have a situation where i need to turn a dictionary entry into a variable. I think i'm close, but am getting a syntax error (quoted at bottom)....can anyone spot whats wrong?

Code:

## Parameters.py

def dictionary_function():
    p = {}
    p['param_1'] = 10
    p['param_2'] = 245
    p['param_3'] = 18
    return p

Code:

## Main.py

from parameters import dictionary_function
p = dictionary_function()

for key in p.keys():
    eval('    %s=%s\n' % (key,p[key]))  # turn the key into variable

I expect to be able to then type "print param_1" and get the value, however, just running the code above, I get the error:

Code:

File "<string>", line 1
    param_1=10
           ^
SyntaxError: invalid syntax

I've tried searching for a solution, but didn't come across anything which helped me

If anyone could spot what i'm doing wrong that would be great! Many thanks!

Sergei Steshenko · 01-31-2011, 09:48 AM

Quote:

Originally Posted by danubuntuman

Hi all,

I have a situation where i need to turn a dictionary entry into a variable. I think i'm close, but am getting a syntax error (quoted at bottom)....can anyone spot whats wrong?

Code:

## Parameters.py

def dictionary_function():
    p = {}
    p['param_1'] = 10
    p['param_2'] = 245
    p['param_3'] = 18
    return p

Code:

## Main.py

from parameters import dictionary_function
p = dictionary_function()

for key in p.keys():
    eval('    %s=%s\n' % (key,p[key]))  # turn the key into variable

I expect to be able to then type "print param_1" and get the value, however, just running the code above, I get the error:

Code:

File "<string>", line 1
    param_1=10
           ^
SyntaxError: invalid syntax

I've tried searching for a solution, but didn't come across anything which helped me

If anyone could spot what i'm doing wrong that would be great! Many thanks!

I am not a Python guy (a Perl one) ; you typically do not need what you want in the first place; if you insist, all kinds of 'eval' functions (it's its Perl name, in Python there is a number of similar functions, I don't remember their names) are your friend.

The fundamental idea is that you convert your keys into source code with variables' names derived from your keys, and then through 'eval' you compile the autogenerated code with new variables and link it (this all is "atomic", i.e. one action) with the running instance of your script.

Julian Andrews · 01-31-2011, 10:31 AM

Hi Sergei,

I think your trouble is because you're using eval() where you should be using exec(). eval() only can evaluate expressions - not statements like an assignment. (You can think of a statement as something which would be ok in the condition for an if clause.

Code:

if param1=10: print 'yay'!

produces a syntax error.

Try the following:

Code:

for k, v in d.iteritems():
    exec("%s = %s" % (k, v))

Note that using exec (or eval) can create a substantial security risk if you don't have complete control over the inputs, so it is generally discouraged if you can find another way to do what you're trying to do.

danubuntuman · 01-31-2011, 10:36 AM

Thanks a lot for your help Julian and Sergei, Exec() did indeed work.

There shouldn't be a security risk for the application of my code really, its a simulation and just needed this to work around a problem.

Thanks for your help!

Sergei Steshenko · 01-31-2011, 11:50 AM

Quote:

Originally Posted by Julian Andrews

Hi Sergei,

I think your trouble is because you're using eval() where you should be using exec(). eval() only can evaluate expressions - not statements like an assignment. (You can think of a statement as something which would be ok in the condition for an if clause.

Code:

if param1=10: print 'yay'!

produces a syntax error.

Try the following:

Code:

for k, v in d.iteritems():
    exec("%s = %s" % (k, v))

Note that using exec (or eval) can create a substantial security risk if you don't have complete control over the inputs, so it is generally discouraged if you can find another way to do what you're trying to do.

In Perl 'exec' is UNIX 'exec'; I have no trouble since I'm not using Python; I was talking about meaning (and explained what I meant), not about the exact names.

bgeddy · 01-31-2011, 01:48 PM

Quote:

Note that using exec (or eval) can create a substantial security risk if you don't have complete control over the inputs, so it is generally discouraged if you can find another way to do what you're trying to do.

Possibly a safer way to do this when global variables are required avoiding the horrible exec:

Code:

for k, v in d.iteritems():
   globals()[k]=v

or when locals variables:

Code:

for k, v in d.iteritems():
    locals()[k]=v

Although still somewhat prone to security risks.