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Print and string formatting

One of the most tried and true debugging tools is print.

Make liberal use of print to output text from your programs that would be useful for troubleshooting and monitoring progress.

Simple use case

You can pass any value to print that can be represented as a string

Most types of data in Python can be represented as a string.

print('Hello, World!')
print(1)
print(3.14)
print(True)
print(None)
print([ 1, 2, 3 ])

String formatting

Format strings allow you to create more complex strings containing plain text and existing variables.

Read the documentation

String formatting is a whole mini-language within the Python language itself. I suggest referring to the official documentation for more.

Basic example

You define a format string like a any other string, except you must prefix the string with a f. Within the format string, you can use curly braces {} to reference a variable

a = 'World'
print(f'Hello, {a}!')

In this example, Python will substitute the placeholder {a} with the value from the variable a which will result in the string Hello, World!.

Real world example

Format strings are sometimes used to dynamically build shell commands. For example

files = [
    ['subject_a.dcm', 'subject_a.nii.gz'],
    ['subject_b.dcm', 'subject_b.nii.gz'],
    ['subject_c.dcm', 'subject_c.nii.gz']
]

for infile, outfile in files:
    command = f'mri_convert {infile} {outfile}'
    print(command)

This will print the following shell commands 

mri_convert subject_a.dcm subject_a.nii.gz
mri_convert subject_b.dcm subject_b.nii.gz
mri_convert subject_c.dcm subject_c.nii.gz

Rounding floats

It's common for a developer to specify how a float value should be displayed within a formatted string. To do this, you can add a format specifier to your placeholder using {variable:specifier} syntax.

For example, if you want your format string to round the variable pi to 4 decimal places, you would append 0.4f to pi

pi = 3.1415926
print(f'π rounded to 4 decimal places is {pi:0.4f}')

For more information on format specifiers, refer to the official documentation here.