What is the ":" symboled mean used in this way in python

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1

I have never seen the “:” symbol used in this way, could someone please tell me what it is and the documentation for it? in

    """
    Performs gradient descent to fit w,b. Updates w,b by taking 
    num_iters gradient steps with learning rate alpha
    
    Args:
      x (ndarray (m,))  : Data, m examples 
      y (ndarray (m,))  : target values
      w_in,b_in (scalar): initial values of model parameters  
      alpha (float):     Learning rate
      num_iters (int):   number of iterations to run gradient descent
      cost_function:     function to call to produce cost
      gradient_function: function to call to produce gradient
      
    Returns:
      w (scalar): Updated value of parameter after running gradient descent
      b (scalar): Updated value of parameter after running gradient descent
      J_history (List): History of cost values
      p_history (list): History of parameters [w,b] 
      """
    
    w = copy.deepcopy(w_in) # avoid modifying global w_in
    # An array to store cost J and w's at each iteration primarily for graphing later
    J_history = []
    p_history = []
    b = b_in
    w = w_in
    
    for i in range(num_iters):
        # Calculate the gradient and update the parameters using gradient_function
        dj_dw, dj_db = gradient_function(x, y, w , b)     

        # Update Parameters using equation (3) above
        b = b - alpha * dj_db                            
        w = w - alpha * dj_dw                            

        # Save cost J at each iteration
        if i<100000:      # prevent resource exhaustion 
            J_history.append( cost_function(x, y, w , b))
            p_history.append([w,b])
        # Print cost every at intervals 10 times or as many iterations if < 10
        if i% math.ceil(num_iters/10) == 0:
            print(f"Iteration {i:4}: Cost {J_history[-1]:0.2e} ",
                  f"dj_dw: {dj_dw: 0.3e}, dj_db: {dj_db: 0.3e}  ",
                  f"w: {w: 0.3e}, b:{b: 0.5e}")
 
    return w, b, J_history, p_history #return w and J,w history for graphing```
2

Colon guides that the lines after that needs to be indented.

3

Hello,

In which way? to which column you are referring to? In the code, there are different “:” used with different meanings.

Best

4

Sorry for not being specific,
Specifically this one

print(f"Iteration {i:4}: Cost {J_history[-1]:0.2e} ",
                  f"dj_dw: {dj_dw: 0.3e}, dj_db: {dj_db: 0.3e}  ",
                  f"w: {w: 0.3e}, b:{b: 0.5e}")

thanks

5

Hello,

no problem, according to the python doc for I/O formatting:
Passing an integer after the ':' will cause that field to be a minimum number of characters wide. This is useful for making columns line up.

basically the “:” inside the “{}” give the format you want to print the variable which precedes the “:”.
More info in this documentation about formatted string literals.

Best,
Davide