Computer languages, like human languages, have a lexical structure. A source code of a Python program
consists of tokens. Tokens are atomic code elements. In Python, we have comments, variables,
literals, operators, delimiters, and keywords.
Comments are used by humans to clarify the source code. All comments in Python language
follow the # character.
#!/usr/bin/python
# Author Jan Bodnar
# ZetCode 2023
def main():
print("Comments example")
main()Everything that follows the # character is ignored by the Python interpreter.
A variable is an identifier which holds a value. In programming we say that we assign
a value to a variable. Technically speaking, a variable is a reference to a computer
memory where the value is stored. In Python language, a variable can hold a string,
a number, or various objects like a function or a class. Variables can be assigned
different values over time.
Variables in Python can be created from alphanumeric characters and the underscore _ character.
A variable cannot begin with a number. This way the Python interpreter can easier
distinguish between a number and a variable.
Value
value2
company_name
These are valid identifiers.
12Val
exx$
first-name
These are examples of invalid identifiers.
The variables are case sensitive. This means that Price, price, and PRICE are three different identifiers.
#!/usr/bin/python
number = 10
Number = 11
NUMBER = 12
print(number, Number, NUMBER)In our script, we assign three numeric values to three identifiers. While it is possible to define three
variables differing in their case, it is not considered a good programming practise.
10 11 12
A literal is any notation for representing a value in a Python source code. Technically, a literal
is assigned a value at compile time, while a variable is assigned at runtime.
age = 29
nationality = "Hungarian"
Here we assign two literals to variables; number 29 and string "Hungarian" are literals.
#!/usr/bin/python
name1 = "Jane"
age1 = 12
name2 = "Rose"
age2 = 16
"Patrick"
34
"Luke"
23
print(name1, age1)
print(name2, age2)If we do not assign a literal to a variable, there is no way how we can work with it. It is dropped.
$ ./main.py
Jane 12
Rose 16
An operator is a symbol used to perform an action on some value.
+ - ~ * ** / //
% << >> & | ^
and or not in not in
is is not < > !=
== <= >=
This is a list of operators available in Python language.
Indentation is used to delimit blocks in Python. Where other programming languages use
curly brackets or keywords such as begin, end, Python uses white space. An increase in
indentation comes after certain statements; a decrease in indentation signifies the
end of the current block. The Python style guide recommends using four spaces per indentation level.
if age > 18:
print("adult person")
for i in range(5):
print(i)After the if keyword a code block is expected. A new statement is started on a new line, indented
with four space characters. The indentation for the following for keyword is decreased back to
the initial one. The for keyword starts a new code block, where its statement(s) are indented.
A delimiter is a sequence of one or more characters used to specify the boundary between separate,
independent regions in plain text or other data stream.
( ) [ ] { }
, : . ` = ;
+= -= *= /= //= %=
<= |= ^= >>= <<= **=
' " \ @
Delimiters are used in various area of the Python language. They are used to build expressions,
string literals, tuples, dictionaries, or lists.
A keyword is a reserved word in the Python programming language. Keywords are used to perform
a specific task in a computer program. For example, import other code, do repetitive tasks or
perform logical operations. A programmer cannot use a keyword as an ordinary variable.
and del global not with
as elif if or yield
assert else import pass False
break except in raise None
class finally is return True
continue for lambda try
def from nonlocal while
This is a list of Python keywords.