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Type Conversions

  • In many circumstances we want to be able to perform operations on differing types, as in the earlier example where we tried to add the number ‘5’ to a string variable. In this situation what we really meant was take a string representation of the number 5 and join that together with our first string. This is called a type conversion and for the string case it is achieved with the str(), e.g.

x = 'The meaning of life is ... '
answer = 42
y = x + str(answer)  # This converts the number 42 to a string and joins
                     # it with the first string and then assigns y to a
                     # new string containing the concatenated string

Printing

  • The most useful situation for this function is when printing output to a screen. Printing is achieved with the print command that expects a string, e.g.

  • in Python 2, you do not need to provide the brackets around the arguments of the print function, but we have included them here, so these examples will work with either python 2 or 3.

print('The meaning of life is ... ')

  • By default the print command outputs a new line to the screen. To suppress this behaviour add a comma after the string:

# this now means the next print statement will continue from where this
# left the cursor
# In python 2 (in this case you cannot include the brackets)
print 'The meaning of life is ... ',
# the equivalent in python 3 is
#print('The meaning of life is ... ', end=' ')

  • The comma can also be used to print several things to the screen on one line separated by a space:

x = 5
y = 6
# Python 2
print "X,Y:",str(x),str(y)   # prints 'X,Y: 5 6' with a newline
#print("X,Y:",str(x),str(y))   # prints '('X,Y:', '5', '6')' with a newline
# Python 3
#print ("X,Y:",str(x),str(y))  # prints 'X,Y: 5 6' with a newline

  • As above printing other types is then simple with the str() function:

x = 'The meaning of life is ... '
answer = 42
print(x + str(answer))

Gives the output:

The meaning of life is ... 42

  • If you want to avoid wrapping variables in str, you can use the format statement to insert values into a template string:

answer = 42
print('The meaning of life is ... {}'.format(answer))

Gives the output:

The meaning of life is ... 42

Converting Between Types

  • Type conversions are not only important for converting to strings but are sometimes necessary to achieve expected answers, e.g.

x = 1/2
print(x)    # Prints 0!!! in Python 2 and 0.5 in Python 3

  • In this case we have asked Python to take two integers (1,2) and then divide them and assign the result to x. The result is another integer which in this case is the integer part of the real number 0.5. If, as in this case, the real number is required then we must ask Python to use floating point numbers instead of integers. This can be achieved in two ways:

x = 1.0/2.0
print(x)

# or using the float function float()
x = 1
y = 2
print(float(x)/float(y))

Gives the output:

0.5
0.5

  • The type conversion functions for the 4 basic types are:

Type

Function

Example

integer

int()

int(3.14159) => 3

float

float()

float(5) => 5.0

bool

bool()

bool(5) => True

string

str()

str(5) => ‘5’

  • If a type cannot be converted then a ‘ValueError’ occurs (see error handling section).

Category:Tested Examples Category:Tested Examples