How to Use exec() in Python - Everything You Need to Know
Tags: python, functions, programming, tutorial
Published: 2023-07-30 08:21:25

The `exec()` function in Python allows us to execute the block of Python code from a string. This built-in function in Python can come in handy when we need to run the **dynamically generated Python code** but it is recommended not to use it carelessly due to some security risks that come with it.

In this tutorial, we'll be going to learn

* how to work with Python's `exec()` function
    
* how to execute Python code using the `exec()` function with code examples
    
* executing Python code from a string and Python source file
    
* using `globals` and `locals` parameters
    

## Python's exec() function

Python's `exec()` function allows us to execute any piece of Python code no matter how big or small that code is. This function helps us to execute the **dynamically generated code**.

Think of a Python interpreter that takes the piece of code, processes it internally, and executes it, the `exec()` function does the same. It is like an independent Python interpreter.

The `exec()` is capable of executing simple Python programs as well as fully featured Python programs. It can execute function calls and definitions, class definitions and instantiations, imports, and much more.

### Syntax

```python
exec(object [ , globals [ , locals]])
```

* `object` - It must be a **string** or a **code object**. If it is a string, it is parsed as *a suite of Python statements* which is executed unless a syntax error occurs. If it is a code object then it will simply execute.
    
* `globals` and `locals` - This allows us to provide dictionaries representing the global and local namespaces.
    

### Return value

The return value of the ***exec()*** function is **None**. It may be because every piece of code doesn't have the final result.

### Initial glance

Here's an initial look at the working of the `exec()` function.

```python
obj = ["apple", "cherry", "melon", "strawberry"]
code = "print([sliced[:4] for sliced in obj if 'a' not in sliced])"

exec(code)

.........
['cher', 'melo']
```

Another example of using the `exec()` function

```python
# The code will continuously run and we can run our code as we do in 
# Python interpreter 
while True:
    exec(input(">>> "))
```

```bash
>>> print("Welcome to GeekPython")
Welcome to GeekPython

>>> import numpy as np
>>> print(np.random.randint(16, size=(2, 4)))
[[11 13  3 13]
 [ 7  6 15  5]]

>>> x = ["apple", "banana", "cherry", "mango"]
>>> print([fruit for fruit in x if "a" not in fruit])
['cherry']
```

It works exactly like a Python interpreter, taking our code, processing it internally, executing it, and returning the correct results.

We're running an infinite loop, and inside it, we're taking input from the command line and wrapping it in the `exec()` function to execute it.

## Executing code from a string input

We can use the `exec()` function to execute the code which is in a string format. There are multiple ways that we can use to build the string-based input:

* **Using one-liners code**
    
* **Using new line characters**
    
* **Using triple-quoted strings with proper formatting**
    

### Using one-liner string-based input

In Python, single-line code, also known as one-liner code, is code written in a single line that can perform multiple tasks at the same time.

If we wrote a single line of Python code, it would look like this:

```python
obj = ["apple", "cherry", "melon", "strawberry"]

print([sliced[:4] for sliced in obj if 'a' not in sliced])
```

**Output**

```shell
['cher', 'melo']
```

But if we run the above code using the `exec()`, the code would be

```python
obj = ["apple", "cherry", "melon", "strawberry"]

exec("print([sliced[:4] for sliced in obj if 'a' not in sliced])")
#-----------------------------OR--------------------------------
exec("code = [sliced[:4] for sliced in obj if 'a' not in sliced]")
```

The other code we wrote above will return nothing if we execute it, instead, the output will be stored in the `code` variable for later access.

### Executing multiple lines of code separated by new line characters

We can combine multiple statements in a single-line string using the new line characters `\n`.

```python
exec("square = int(input('Enter the number: '))\nprint(f'The square of {square}:', square**2)")
```

**Output**

```shell
Enter the number: 30
The square of 30: 900
```

A new line character (`\n`) is defined to make the `exec()` function understand our single-line string-based code as a multiline set of Python statements.

### Using triple-quoted string

In Python, we frequently use triple quotes to comment on or document our code. However, in this case, we'll use it to generate string-based input that looks and behaves exactly like normal Python code.

The code we write within triple quotes must be properly indented and formatted, just like normal Python code. See the example below for a better understanding.

```python
sample_code = """

integers = [4, 7, 2, 9, 44]

def square(num):
    return num ** 2

def odd_num(num):
    return num % 2 == 1

square_if_even = [square(number) for number in integers if number % 2 == 0]

odd_number = [number for number in integers if odd_num(number)]

print("Original values:", integers)

print("Square of even number:", square_if_even)

print("Odd number:", odd_number)

"""
exec(sample_code)
```

**Output**

```shell
Original values: [4, 7, 2, 9, 44]
Square of even number: [16, 4, 1936]
Odd number: [7, 9]
```

The above code is similar to standard Python code, with proper indentation and formatting, but it is wrapped within triple quotes, resulting in string-based input stored in the `sample_code` variable, which was then executed using the `exec()` function.

## Executing code from a Python file

We can use the `exec()` function to execute the code from Python(`.py`) source file by reading the content of the file using the [open()](https://docs.python.org/3/library/functions.html#open) function.

Consider the following example, which includes a `sample.py` file containing the following code:

```python
# sample.py
def anime(name):
    print(f"Favourite anime: {name}")

anime("One Piece")

list_of_anime = input("Enter your favourite anime: ").split(",")
print("Your Favourite anime:", list_of_anime)
```

The code simply prints the anime's name here, while the following block of code takes the input of your favorite anime separated by commas and prints the desired output.

**Executing the Python source file using the exec function**

```python
with open("sample.py", mode="r") as sample:
    file = sample.read()

exec(file)
```

**Output**

```shell
Favourite anime: One Piece
Enter your favourite anime: One Piece, Naruto, Demon Slayer, Jujutsu kaisen
Your Favourite anime: ['One Piece', ' Naruto', ' Demon Slayer', ' Jujutsu kaisen']
```

We used the `open()` function using the `with` statement to open the `.py` file as a regular text file and then used the `.read()` on the file object to read the content of the file as a string into the `file` variable which is passed in the `exec` to execute the code.

## Using globals and locals params

These parameters are entirely optional. We can use `globals` and `locals` parameters to limit the use of functions, methods, and variables that aren't required.

Because these parameters are optional, omitting them causes the `exec()` function to execute the input code in the current scope. Consider the following example to better understand it.

```python
code = """
out = str1 + str2
print(out)
"""
# global variables
str1 = "Hel"
str2 = "lo"

exec(code)
```

**Output**

```shell
Hello
```

The code above ran successfully and produced an output that combined both global variables. Because the `globals` and `locals` parameters were not specified, the `exec()` function executed the code input in the current scope. The current scope is **global** in this case.

Here's an example of how to get the value of a variable in the current scope of code.

```python
code = """
out = str1 + str2
print(out)
"""
# global variables
str1 = "Hel"
str2 = "lo"

print(out)
```

```python
Traceback (most recent call last):
    .....
NameError: name 'out' is not defined. Did you mean: 'oct'?
```

In the preceding code, we attempted to access the value of the `out` variable before calling `exec()` and received an error.

However, we can access the value of the out variable after calling `exec()` because the variable defined in the code input will be available in the current scope after calling `exec()`.

```python
exec(code)
print(out)
```

**Output**

```shell
Hello
Hello
```

**Using** `globals` and `locals` parameters

```python
code = """
out = str1 + str2
print(out)
"""
# global variables
str1 = "Hel"
str2 = "lo"

exec(code, {"str1": str1})
```

**Output**

```python
Traceback (most recent call last):
    ....
NameError: name 'str2' is not defined. Did you mean: 'str1'?
```

The code returns an error because we didn't define the key holding the `str2` in the dictionary, so the `exec()` doesn't have access to it.

```python
code = """
out = str1 + str2
print(out)
"""
# global variables
str1 = "Hel"
str2 = "lo"

exec(code, {"str1": str1, "str2": str2})

print(out)
```

**Output**

```python
Hello
Traceback (most recent call last):
     ....
NameError: name 'out' is not defined. Did you mean: 'oct'?
```

The `exec()` function now has access to both global variables, and the code returns the output without error, but we didn't have access to the out after the call to `exec()` this time because we're using the custom dictionary to provide an execution scope to the `exec()`.

Here's an example of using `locals` and `globals` together

```python
code = """
out = str1 + str2 + " " + x
print(out)
"""
# global variables
str1 = "Hel"
str2 = "lo"

def local():
    # local variable
    x = "there"
    exec(code, {"str1": str1, "str2": str2}, {"x": x})

local()
```

**Output**

```shell
Hello there
```

In the above code, we called `exec()` from within the local function. In the global scope, we have global variables and a local variable in the local scope (function level). The globals parameter specifies the variables `str1` and `str2`, while the locals parameter specifies the variable `x`.

### Blocking unnecessary methods and variables

Using the `globals` and `locals` params, we can control whether to restrict or use any variable or method in our code input. In this section, we'll limit some of the functions in Python's `datetime` module.

```python
from datetime import *

code = """
curr_time = datetime.now()
print(curr_time)
"""
exec(code, {})
```

**Output**

```python
Traceback (most recent call last):
     ....
NameError: name 'datetime' is not defined
```

We restricted the use of the `datetime` method from the `datetime` module, which resulted in an error.

### Using necessary methods and variables

We can only use the methods that are required with the `exec()`.

```python
from datetime import *

# Allowing only two methods
allowed_param = {'datetime': datetime, 'timedelta': timedelta}

exec("print(datetime.now())", allowed_param)
exec("print(datetime.now() + timedelta(days=2))", allowed_param)

# date() method is not allowed
exec("print(date(2022, 9, 4))", allowed_param)
```

**Output**

```python
2022-10-15 18:40:44.290550
2022-10-17 18:40:44.290550

Traceback (most recent call last):
     ....
NameError: name 'date' is not defined
```

The error occurred because the `date` method was not permitted. Except for two methods, `datetime` and `timedelta`, all methods in the `datetime` module were forbidden.

Let's see what else we can accomplish with `globals` and `locals` parameters.

```python
from datetime import *

# Setting globals parameter to __builtins__
globals_param = {'__builtins__': __builtins__}

# Setting locals parameter to take only print(), slice() and dir()
locals_param = {'print': print, 'dir': dir, 'slice': slice}

exec('print(slice(2))', globals_param, locals_param)
exec('print(dir())', globals_param, locals_param)
```

**Output**

```python
slice(None, 2, None)
['dir', 'print', 'slice']
```

Inside the `exec()` function, only the `slice()` method and all built-in methods can be executed. Even though the `slice()` method is not from the `datetime` module, it works perfectly here.

We can also limit the use of `__builtins__` by setting it to `None`.

```python
from datetime import *

# Setting globals parameter to none
globals_param = {'__builtins__': None}

# Setting locals parameter to take only print(), slice(), sum() and dir()
locals_param = {'print': print, 'dir': dir, 'slice': slice, 'sum': sum}

# Allowed methods directory
exec('print(dir())', globals_param, locals_param)
exec('print(f"Sum of numbers: {sum([4, 6, 7])}")', globals_param, locals_param)
```

**Output**

```python
['dir', 'print', 'slice', 'sum']
Sum of numbers: 17
```

We limited the use of `__builtins__`, so we can't use the built-in methods and can only execute the `print`, `sum`, `slice`, and `dir` methods inside the `exec()`.

## Conclusion

We've learned how to use the built-in Python `exec()` function to execute code from a string-based input. It allows you to run dynamically generated Python code.

The topics we've learned

* what is `exec()` function and working with it
    
* executing Python code from a string-based input and Python source files
    
* understanding the use of the `globals` and `locals` parameters
    

Additionally, we've coded some of the examples which helped us understand the `exec()` function better.

---

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---

**That's all for now**

**Keep Coding✌✌**