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pythonpython-3.xsyntaxundefinedeval

String is being treated as variable in `eval` statement

I'm trying to do an extremely simple pass-in of a string variable into my eval statement. However, my string is being treated as an undefined variable.

Here is my code:

condition = 'hi'
print(eval("2 + 4 * len(%s)" % (condition)))

Output:

Traceback (most recent call last):
  File "C:\test.py", line 3, in <module>
    print(eval("4 + 3 * len(%s)" % (condition)))
  File "<string>", line 1, in <module>
NameError: name 'hi' is not defined

However, when I define hi as if it were a variable, all the sudden the code compiles and runs:

condition = 'hi'
hi = 'hi'
print(eval("2 + 4 * len(%s)" % (condition)))

Output:

10

What in the world? This seems totally unintuitive to me. Could someone help me define condition in a way that Python does not ask for 'hi' to be defined as well?

Solution

  • You need quotes around %s, like so:

    condition = 'hi'
print(eval("2 + 4 * len('%s')" % (condition)))

    This way you are passing len() a string 'hi' instead of a variable hi.