How do I put a variable’s value inside a string (interpolate it into the string)?

Detailed guide on string interpolation

Exploiting the full potential of f-strings

In f-strings, you can integrate expressions directly:

distance = 15
# And now, for a bit of Python humor...
message = f"Walked {distance*2} miles today. Think I've earned myself a pythonic pie!"
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Note that with f-strings, even complex operations can be included directly into your strings, marrying functionality with clarity.

Concatenation: Mind your datatypes!

Before using string concatenation, always convert non-string types:

version = 3.10
# Python, always keeping us on our toes.
details = "Welcome to Python " + str(version) + "! Don't confuse it with room number."
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Leaving out the str() conversion results in a TypeError, because Python strictly requires the same datatypes for operations like these.

% operator: The grandparent of formatting in Python

Classic Python string formatting with % operator involves conversion specifier:

gpa = 3.85
# The '%' operator - vintage swag infused!
academic_record = "Your GPA is %.2f. No, not Grilled Python Aroma!" % gpa
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Here %.2f signifies the number is formatted as a float having two decimal places. The % method is a versatile age-old trick.

locals(): A smoother interpolation ride

For painless interpolation when dealing with heaps of variables, **locals() comes to our rescue!

weapon = "slingshot"
ammo = "python"
note = "Arm yourself with a {weapon} loaded with a {ammo}!".format(**locals())
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In this way, we don't need to pass each variable separately, which makes this method particularly elegant for a big list of variables.

string.Template: Substitution with safety

In situations prioritizing security, we can use string.Template for a safer form of substitution:

from string import Template
mood = 'pythonized'
beverage = 'Python Potion'
template_string = Template("Feeling ${feeling}? Guzzle some ${drink}.")
final_string = template_string.substitute(feeling=mood, drink=beverage)
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Template substitution averts potential security issues that might arise from other string formatting methods.

Must-know tips for formatting strings

Padding and aligning with f-strings

You can easily introduce inline formatting in f-strings. For example, if you need to pad numbers with zeros:

page_number = 5
formatted_page = f"Page No.{page_number:03}" # Yields Page No.005 (Now, that's formatted!)
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Managing numerous files with naming in loops

Here are some methods to manage dynamically named files in loops:

for i in range(5):
    filename_f = f"file_{i}.txt"     # f-string way
    filename_format = "file_{}.txt".format(i)  # format() way
    filename_percent = "file_%d.txt" % i  # % formatting way. Oldie but goodie!
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These techniques make file handling a piece of cake, oops, Python pie!

Steer clear of inappropriate keys

Do remember that Python raises KeyError or ValueError if a mismatch in placeholders happens. Prepare for some debugging action!