This can be handy when you need to evaluate the same expression multiple times. If you call Python’s eval() using a compiled-code-based input, then the function performs the evaluation step and immediately returns the result. If you use compile() to compile the expressions that you’re going to pass to eval(), then eval() goes through the following steps: If you want to process the compiled code with eval(), then this argument should be set to "eval". mode specifies which kind of compiled code you want to get.If you’re going to use a string-based input, then the value for this argument should be "". filename gives the file from which the code was read.This argument accepts normal strings, byte strings, and AST objects. source holds the source code that you want to compile.The details of how to use compile() are beyond the scope of this tutorial, but here’s a quick look at its first three required arguments: This is a built-in function that can compile an input string into a code object or an AST object so that you can evaluate it with eval(). To compile the code that you’re going to pass to eval(), you can use compile(). You can also pass compiled code objects to Python’s eval(). In the above example, you try to evaluate an incomplete expression ( "5 + 7 *") and get a Synta圎rror because the parser doesn’t understand the syntax of the expression. You can’t pass an expression to eval() that violates Python syntax. > # Incomplete expression > eval ( "5 + 7 *" )įile "", line 1 5 + 7 * ^ Synta圎rror: unexpected EOF while parsing Take a look at the following example in which you try to execute an if statement using eval(): If you try to pass a compound statement to eval(), then you’ll get a Synta圎rror. A statement is either an expression or one of several constructs with a keyword, such as if, while or for. On the other hand, a Python statement has the following definition:Ī statement is part of a suite (a “block” of code). Assignments are also statements, not expressions. There are also statements which cannot be used as expressions, such as while. In contrast to many other languages, not all language constructs are expressions. In other words, an expression is an accumulation of expression elements like literals, names, attribute access, operators or function calls which all return a value. The Python documentation defines expression as follows:Ī piece of syntax which can be evaluated to some value. The name expression for the first argument to eval() highlights that the function works only with expressions and not with compound statements. To evaluate a string-based expression, Python’s eval() runs the following steps: By default, eval() has access to global names like x in the above example. When you call eval() with a string as an argument, the function returns the value that results from evaluating the input string. If you want to get the code for this application, then you can click on the box below: With this example, you’ll apply everything you’ve learned about eval() to a real-world problem. How eval() can make your code insecure and how to minimize the associated security risksĪdditionally, you’ll learn how to use Python’s eval() to code an application that interactively evaluates math expressions.How to use eval() to dynamically evaluate arbitrary string-based or compiled-code-based input.In this tutorial, you’ll learn how eval() works and how to use it safely and effectively in your Python programs. This function can be handy when you’re trying to dynamically evaluate Python expressions from any input that comes as a string or a compiled code object.Īlthough Python’s eval() is an incredibly useful tool, the function has some important security implications that you should consider before using it. Python’s eval() allows you to evaluate arbitrary Python expressions from a string-based or compiled-code-based input. Watch it together with the written tutorial to deepen your understanding: Evaluate Expressions Dynamically With Python eval() Watch Now This tutorial has a related video course created by the Real Python team.
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