Python conditional print formatting

Question

I have a function like this:

def PrintXY(x,y):
    print('{:<10,.3g} {:<10,.3g}'.format(x,y) )

When I run this it's perfect:

>>> x = 1/3
>>> y = 5/3
>>> PrintXY(x,y)
0.333      1.67

But let's say that x and y are not guaranteed to exist:

>>> PrintXY(x, None)
unsupported format string passed to NoneType.__format__

In that case I'd like to print nothing, just empty space. I tried:

def PrintXY(x,y):
    if y is None: 
        y = ''
    print('{:<10,.3g} {:<10,.3g}'.format(x,y) )

But that gives:

ValueError: Unknown format code 'g' for object of type 'str'

How can I print whitespace if the number doesn't exist, and proper formating when the number does exist? I'd rather not print 0 or -9999 to signify an error.

Answer 1

I have separated it out to make it clear what the statements achieve. You can combine this into one line but it would make the code a bit harder to read

def PrintXY(x,y):
    x_str = '{:.3g}'.format(x) if x else ''
    y_str = '{:.3g}'.format(y) if y else ''
    print('{:<10} {:<10}'.format(x_str, y_str))

Then running gives

In [179]: PrintXY(1/3., 1/2.)
     ...: PrintXY(1/3., None)
     ...: PrintXY(None, 1/2.)
     ...:
0.333      0.5
0.333
           0.5

Another alternative to make sure your format remains consistent is to do

def PrintXY(x,y):
    fmtr = '{:.3g}'
    x_str = fmtr.format(x) if x else ''
    y_str = fmtr.format(y) if y else ''
    print('{:<10} {:<10}'.format(x_str, y_str))

Answer 2

You could try this:

def PrintXY(x=None, y=None):        
    print(''.join(['{:<10,.3g}'.format(n) if n is not None else '' for n in [x, y]]))

This you could easily expand to use x , y and z .

Answer 3

You could just use different print commands like this:

def PrintXY(x,y):
    if y is None: 
        print('{:<10,.3g}'.format(x) )
    else:
        print('{:<10,.3g} {:<10,.3g}'.format(x,y) )

Answer 4

You can make code more readable and easy to understand the condition that you have in your problem statement, you can try this also:

def PrintXY(x,y):
    formatter = None

    if x is None and y is None:
        x, y = '', ''
        formatter = '{} {}'
    if x is None:
        y = ''
        formatter = '{} {:<10,.3g}'
    if y is None:
        x = ''
        formatter = '{:<10,.3g} {}'
    else:
        formatter = '{:<10,.3g} {:<10,.3g}'

    print(formatter.format(x,y))