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Appendix 1
Simplified language reference
Keywords
The following table gives basic descriptions of the Python keywords, which are a reserved
part of the language and so cannot be used for variable names.
Keyword
Description
Example
and
Performs the logical AND
operation (conjunction) on two
values, yielding a true value if both
are true, and otherwise a false
value.
if x and y:
print("Both are true")
as
Used in conjunction with the with
keyword to define a variable that
holds a context manager object.
Alternatively used with import
statements to create synonyms for
external components.
[See examples for ‘with’ and
‘import’]
assert
Checks whether a statement is true,
and if not triggers an exception.
Often used to check the validity of
input values. The command is
ignored if Python is run in
optimised mode.
assert(isinstance(x, int))
Note: triggers an
AssertionError exception if the
test fails.
break
Causes the program execution
within current, innermost, iterative
loop (invoked with for or while) to
stop immediately.
for value in valueList:
if value > 0:
print('Loop stopped')
break
class
Defines a class: the construction of
a named type of Python object,
which may be based on one or more
other classes.
class
Biologist(Scientist):
field = 'Biology'
continue
Causes program execution within
current, innermost, iterative loop
for value in valueList:
(invoked with for or while) to skip
to the next iteration.
if value > 0:
print('Value skipped')
continue
def
Defines a named Python function
(subroutine), the code for which is
in an indented block.
def dotProduct(vec1,
vec2):
pairs = zip(vec1, vec2)
p = sum([a*b for a,b in
pairs])
return p
del
Deletes a variable or item, directly
from memory or from an enclosing
data structure.
p =
[‘Neptune’,‘Uranus’,‘Pluto’]
del p[2]
Note: deleted list item with
index 2 (Pluto).
elif
Used after an if statement to define
a further block of code that is
conditionally executed if a different
condition holds.
if x > 0:
print("x is positive")
elif x < 0:
print("x is negative")
else
Used after an if statement to define
a final block of code that is
executed if none of the conditions
after the if or elif statements hold.
Also used after try to define a block
that is executed if no error occurs,
and after for to define a block that
is executed if the for loop has not
been exited because of a break.
if x % 2:
print("x is odd")
else:
print("x is even")
except
Used after the try keyword to
provide a code block that catches
and deals with particular kinds of
error exception.
[See example for try]
exec
Executes Python statements that are
contained within text strings or a
file; allows the creation of dynamic
code.
text = ‘sum(range(1,100,3))’
exec(text)
finally
Used after the try keyword in order
to specify a block of code that is
always executed after exception
fileObj = open(name)
try:
handling, typically to perform
clean-up operations.
print(fileObj.readlines())
finally:
fileObj.close()
for
Creates a repeating, iterative loop
where a variable is assigned values
in turn from a collection or iterable
object.
total = 0.0
for value in
[1,3,5,7,9,11]:
total += value ** 0.5
from
Used in a statement to specify
which module to import a
component from.
from math import sqrt
y = sqrt(2.0)
global
Makes named variables accessible
in a global context, i.e. throughout
the whole file, even if they only
appear within inner, local code
blocks.
def func(text):
global value
value = len(text)
func('hello')
print(value) # 5
if
Used to execute a block of code
only on condition that a statement
holds. May be used in conjunction
with elif and else.
if x > 0:
print("Here if x is
positive")
import
Imports a named external module
so that it is available within the
current program. Can be used with
from to import a particular sub-
component and with as to create a
synonym for the module.
import math
y = math.sqrt(2.0)
import numpy as np
a = np.array([2,0,4,1])
in
Used in various situations. On its
own used in tests whether an item is
a member of a collection, giving
True if it is and False otherwise.
Also used after for and exec
keywords.
letters =
['G','C','A','T']
if x in letters:
print('x is in the
list')
[See also for and exec
keywords]
is
Compares two Python objects to
test whether they are the same
object, yielding True if they are and
False otherwise.
if x is None:
print("x not defined")
is not
Compares two Python objects to
test whether they are not the same
object, yielding True if they are
different and False if they are the
same object.
if x is not None:
print("x is defined")
lambda
Creates an unnamed (anonymous)
function. Useful for defining small
functions in place in larger
expressions.
data = [('A',4), ('B',9),
('C',2)]
data.sort(key=lambda x:
x[1])
Sorts on the second item.
not
Performs a logical NOT operation
(negation) on a value, giving True if
the value is false and False if the
value is true.
if not x:
print("x is false")
or
Performs the logical OR operation
(disjunction) on two values,
yielding a true value if one or other
is true, and otherwise a false value.
if x or y:
print("At least one is
true")
pass
A keyword that doesn’t actually do
anything except form part of a valid
Python syntax. Used as a
placeholder where code may be
filled in later.
if x is None:
pass # Add code later
else:
x += 2.0
print
Displays a line of text to screen
(standard out), with an option to
redirect the data, e.g. to a file. In
Python 2 print is a statement or a
function. In Python 3 it is a
function.
print ‘Hello’
print >> fileObj, “Data line”
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