Variables, Strings and Numbers

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Order number, amount, status — a program constantly holds small pieces of data, and every piece gets a name. That name is a variable. The second thing you learn on day one: text and numbers look alike and behave differently.

A variable is a name for a value

live example

order_id = "ord-042"
status = 200
print(order_id, status)

Name on the left, value on the right, an equals sign between them. It is not the "equals" of mathematics but "put the right side into the left".

The value can be replaced — that is what makes it a variable:

live example

status = 200
status = 404
print(status)        # 404

Practice here: Print a string · Define a variable

Numbers and strings are different things

The quotes decide everything. "200" is a string, three characters of text. 200 is a number you can do arithmetic with:

live example

code = "200"
print(code + "1")    # 2001 — strings are glued together
code = 200
print(code + 1)      # 201 — numbers are added

A value almost always arrives as text — from a file, from a form, from a service response — and arithmetic needs numbers. int() and float() do the conversion:

live example

total = int("4990")        # 4990, now a number
price = float("99.90")     # 99.9, a decimal
print(total + 10, price)   # 5000 99.9

The other way round, from number to text, is str():

live example

text = str(4990)
print(text + " RUB")       # 4990 RUB — glued as text

int("four") fails — and rightly so: there is nothing to convert.

Practice here: Number from a string

Strings: the daily material

Quotes around a string are single or double — take the ones that do not appear inside the text itself. len() gives the length:

live example

print(len("ord-042"))     # 7

A message is assembled from pieces with an f-string: put an f before the quotes and write the expression in curly braces. Addition works too, but only between strings — a number cannot be plugged in that way.

live example

# error: wrong way — a number cannot be glued to a string
order_id = "ord-042"
total = 4990
print("Order " + order_id + " for " + total)

The program stops with TypeError: can only concatenate str (not "int") to str, that is, "a string can only be glued to a string".

live example

# right way
order_id = "ord-042"
total = 4990
print(f"Order {order_id} for {total} RUB")

Inside the braces you can calculate: f"twice {total * 2}".

Searching inside a string is the in operator:

live example

response = "Order not found"
print("not found" in response)    # True

The answer is True or False. Those two values are called booleans, and every check rests on them.

Five operations you will need all the time:

live example

print("  ord-042  ".strip())          # ord-042 — trim spaces
print("ORD-042".lower())              # ord-042 — lower case
print("ord-042".startswith("ord"))    # True — starts with
print("ord-042".split("-"))           # ['ord', '042'] — cut by a character
print("ord-042".replace("ord", "x"))  # x-042 — replace

strip() and lower() save you when comparing data from different places: one file says "Anna ", another says "anna" — the same thing to the eye, not to the program.

Practice here: Substitute a value into a string · String length · String inside a string

Comments

Everything after a # is a note for a human, the program skips it:

live example

price = 4990        # price before the discount
print(price)

Write them where the code does not explain why: what a line does is visible anyway.

Common mistakes

Unclosed quotes — print("hello). A SyntaxError with the line number; check quote and bracket pairs.

Typo in a name — you defined order_id and used order_di. NameError: name 'order_di' is not defined, that is, "I do not know that name".

Compared a number with a string. The program does not crash, it quietly answers "no match":

live example

# wrong way
code = "200"
print(code == 200)        # False — text compared with a number

live example

# right way
code = "200"
print(int(code) == 200)   # True — both brought to one type first

In short

• A variable is a name for a value; = stores, it does not compare.
• "200" and 200 are different: text becomes a number with int() and float(), a number becomes text with str().
• An f-string (f"Order {order_id}") is the normal way to build a message; a number cannot be glued to a string with +.
• strip() and lower() bring data from different places to a comparable form.
• in answers True or False — every check starts there.

What to read next

• Comparisons and conditions — how a program makes a decision.
• Lists and dicts — what JSON looks like from the inside.
• Loops — how to go through every record and miss nothing.