Unit 3: Files, exceptions and visualization

pythonwith open("notes.txt", "w") as f:      # with closes the file automatically
    f.write("Line 1\n")
with open("notes.txt", "r") as f:
    for line in f:
        print(line.strip())

Pickle saves Python objects to binary files: pickle.dump(obj, f) and obj = pickle.load(f).

pythonimport csv, json
with open("data.csv", newline="") as f:
    for row in csv.reader(f): print(row)
with open("data.json") as f:
    data = json.load(f)                  # json.dump(obj, f) writes

An exception is a run-time error that stops the program unless handled.

pythontry:
    a = int(input()); b = int(input())
    print(a / b)
except ZeroDivisionError:
    print("Cannot divide by zero")
except ValueError:
    print("Enter numbers only")
else:
    print("Division done")               # runs if no exception
finally:
    print("Program finished")            # always runs

raise throws an exception yourself: if age < 0: raise ValueError("Age cannot be negative").

pythonimport matplotlib.pyplot as plt
import numpy as np
x = np.linspace(0, 2 * np.pi, 100)
plt.plot(x, np.sin(x), label="sin")
plt.plot(x, np.cos(x), label="cos")
plt.legend(); plt.title("Sine and cosine"); plt.show()

plt.hist([45, 60, 72, 72, 85, 90], bins=5); plt.show()
plt.pie([40, 35, 25], labels=["BCA", "BBA", "B.Com"], autopct="%1.0f%%"); plt.show()

3-D plots use from mpl_toolkits.mplot3d import Axes3D and ax = plt.axes(projection="3d").