Python Syntax

Reserved keywords

The Python language reserves a small set of keywords that cannot be used as variable names, function names, or any other identifiers:

Keyword	Description
and	A logical operator
as	To create an alias
assert	For debugging
break	To break out of a loop
class	To define a class
continue	To continue to the next iteration of a loop
def	To define a function
del	To delete an object
elif	Used in conditional statements, same as else if
else	Used in conditional statements
except	Used with exceptions, what to do when an exception occurs
False	Boolean value, the result of comparison operations
finally	Used with exceptions, a block of code that will be executed no matter if there is an exception or not
for	To create a for loop
from	To import specific parts of a module
global	To declare a global variable
if	To make a conditional statement
import	To import a module
in	To check if a value is present in a list, tuple, etc.
is	To test if two variables are equal
lambda	To create an anonymous function
None	Represents a null value
nonlocal	To declare a non-local variable
not	A logical operator
or	A logical operator
pass	A null statement, a statement that will do nothing
raise	To raise an exception
return	To exit a function and return a value
True	Boolean value, the result of comparison operations
try	To make a try…except statement
while	To create a while loop
with	Used to simplify exception handling
yield	To end a function returns a generator

Indentation

Python depends on indentation to follow the correct flow of execution. In the example below, we see that there is a possible error as Hello Jack. is being printed before anything else. Many will assume it will be printed after Hello John. as it is directly after it, but indentation wise it isn’t part of the main function.

Example

#!/usr/bin/env python3

def main():
    if 1 > 2:
        print("Hello World.")
    print("Hello John.")

print("Hello Jack.")


if __name__ == "__main__":
    main()

Output

Hello Jack.
Hello John.

If we try to correct this error by adding a space before the print statement, then Python still gives an error as the indentation doesn’t match all other levels. Python is very similar to YAML and all the indentations needs to be the same. See PEP 8#indentation for a complete set of rules about indentation.

Example:

#!/usr/bin/env python3

def main():
    if 1 > 2:
        print("Hello World.")
    print("Hello John.")
print("Hello Jack.")


if __name__ == "__main__":
    main()

Output

  File "/workspaces/mastering-python/hello.py", line 7
    print("Hello Jack.")
                        ^
IndentationError: unindent does not match any outer indentation level

Note

Python has no hard guideline on indentation and while most developer choose to use 4 spaces, the requirements is to have at least 1 space for each level of indentation.

Comments

Any line that starts with # will be ignored by Python.

Example

#!/usr/bin/env python3

def main():
    if 1 > 2:
        print("Hello World.")
    print("Hello John.")
    # print("Hello Jack.")


if __name__ == "__main__":
    main()

Output

Hello John.

Multiline comments can be done by starting every line with the #-sign.

Example

#!/usr/bin/env python3

def main():
    if 1 > 2:
        print("Hello World.")
    print("Hello John.")
    # print("Hello Jack.")
    # print("Hello James.")


if __name__ == "__main__":
    main()

Output

Hello John.

For multiline comments Python also supports comments between triple quotes as in the example below.

Example

#!/usr/bin/env python3

def main():
    if 1 > 2:
        print("Hello World.")
    print("Hello John.")
    """
    print("Hello Jack.")
    print("Hello James.")
    """


if __name__ == "__main__":
    main()

Output

Hello John.