Plot Graph

plot.py

import re

import numpy as np
from matplotlib import pyplot as plt

replacements = {
    'sin' : 'np.sin',
    'cos' : 'np.cos',
    'exp': 'np.exp',
    'sqrt': 'np.sqrt',
    '^': '**',
}

allowed_words = [
    'x',
    'sin',
    'cos',
    'sqrt',
    'exp',
]

def string2func(string):
    ''' evaluates the string and returns a function of x '''
    # find all words and check if all are allowed:
    for word in re.findall('[a-zA-Z_]+', string):
        if word not in allowed_words:
            raise ValueError(
                '"{}" is forbidden to use in math expression'.format(word)
            )

    for old, new in replacements.items():
        string = string.replace(old, new)

    def func(x):
        return eval(string)

    return func


if __name__ == '__main__':

    func = string2func(input('enter function: f(x) = '))
    a = float(input('enter lower limit: '))
    b = float(input('enter upper limit: '))
    x = np.linspace(a, b, 250)

    plt.plot(x, func(x))
    plt.xlim(a, b)
    plt.show()