Module-1.pptx

Python
 Created in 1991 by Guido van Rossum (now at Google)
 Useful as a scripting language
– script: A small program meant for one-time use
– Targeted towards small to medium sized projects
 Used by:
– Google, Yahoo!, Youtube
– Many Linux distributions
– Games and apps (e.g. Eve Online)

InstallingPython
Windows:
 Download Python from
http://www.python.org
 Install Python.
 Run Idle from the Start Menu.
Mac OS X:
 Python is already installed.
 Open a terminal and run
python or run Idle from
Finder.
Linux:
 Chances are you already have Python
installed. To check, run pythonfrom
the terminal.
 If not, install from your
distribution's package system.

Installing Python
• Python distribution is available for a wide variety of platforms.
• Download only the binary code applicable for your platform and
install Python.
Unix and Linux Installation
• Here are the simple steps to install Python on
• Unix/Linux machine.
• Open a Web browser and go to http://www.python.org/download/.
• Follow the link to download zipped source code available for
Unix/Linux, download and extract files.
• Editing the Modules/Setup file if you want to customize some
options.
• run ./configure script, make & make install
• This installs Python at standard location /usr/local/bin and its
libraries at /usr/local/lib/pythonXX where XX is the version of
Python.

Windows Installation
• Here are the steps to install Python on Windows machine.
• Open a Web browser and go to http://www.python.org/download/
• Follow the link for the Windows installer python-XYZ.msi file
• where XYZ is the version you need to install.
• To use this installer python-XYZ.msi, the Windows system must
support Microsoft Installer 2.0. Save the installer file to your
local machine and then run it to find out if your machine
supports MSI.
• Run the downloaded file. This brings up the Python install
wizard, which is really easy to use. Just accept the default
settings, wait until the install is finished, and you are done.

InterpretedLanguage
• interpreted
– Not compiled like Java
– Code is written and then directly executed by an interpreter
– Type commands into interpreter and see immediate results
Code Compiler
Runtime
Environment
Computer
Code Interpreter Computer
Java:
Python:

What sort oflanguageis Python?
Explicitly
compiled
to machine
code
Purely
interpreted
C, C++,
Fortran
Shell,
Perl
Explicitly
compiled
to byte
code
Java, C#
Implicitly
compiled
to byte
code
Python
Compiled Interpreted

SimpleProgramme
• Python does not have a main method like Java
– The program's main code is just written directly in the file
• Python statements do not end with semicolons
– Escape sequences such as "are the same as in Java
– Strings can also start/end with '
helloJNCCE.py
1 Print("Hello, JNCCE!"

Who usesPython?
On-line games
Web services
Applications
Science
Instrument control
Embedded systems

RunningPython― 2
Unix prompt
Unix command
Introductory blurb
Python prompt
3 2012, 15:54:42)
Python version
$ python3
Python 3.2.3 (default, May
[GCC 4.6.3] on linux2
>>>

QuittingPython
>>> exit()
>>> quit()
>>> Ctrl D
+
Any one
of these

A firstPythoncommand
>>> print('Hello, world!')
Hello, world!
>>>
Python prompt
Python command
Output
Python prompt

Pythoncommands
Python “function”
Function’s “argument”
Round brackets
— “parentheses”
print('Hello, world!')
“Case sensitive”
print ≠ PRINT

Expressions
• Python commands are called statements.
•One kind of statement is an expression statement, or expression.
• Like any programming language, Python can evaluate basic
mathematical expressions.
• For example, the following expression adds 4 and 13:
>>> 4 + 13
17

Type int
• It’s not surprising that 4 + 13 is 17.
• For example, look at what happens when we divide 17 by 10:
>>> 17 / 10
1
• You would expect the result to be 1.7, but Python produces 1
instead. This is because every value in Python has a particular type,
and the types of values determine how they behave when they’re
combined..
• In Python, an expression involving values of a certain type
produces a value of that same type. For example, 17 and 10 are
integers—in Python, we say they are of type int. When we divide
one by the other, the result is also an int.

Type float
• Python has another type called float to represent numbers with
fractional parts.
• An expression involving two floats produces a float:
>>> 17.0 / 10.0
1.7
• When an expression’s operands are an int and a float, Python
automatically converts the int to a float.
>>> 17.0 / 10
1.7
>>> 17 / 10.0
1.7
>>> 17 / 10.
1.7
>>> 17. / 10
1.7

What Is a Type?
• In computing, a type is a set of values, along with a set of
operations that can be performed on those values.
• For example, the type int is the values …, -3, -2, -1, 0, 1, 2, 3, …,
along with the operators +, -, *, /, and %.
• 84.2 is a member of the set of float values.
• Look at Python’s version of the fraction 1 ⁄ 3
>>> 1.0 / 3.0
0.33333333333333331
• What’s that 1 doing at the end? Shouldn’t it be a 3? The problem is
that real computers have a finite amount of memory, which limits
how much information they can store about any single number.

Operator Precedence
• Let’s convert Fahrenheit to Celsius. To do this, we subtract 32 from
the temperature in Fahrenheit and then multiply by 5 ⁄ 9:
>>> 212 - 32.0 * 5.0 / 9.0
194.22222222222223
• Python claims the result is 194.222222222222232 degrees Celsius
when in fact it should be 100. The problem is that * and / have
higher precedence than - .
• This means that what we actually calculated was 212 - ((32.0 * 5.0)
/ 9.0).
• We can alter the order of precedence by putting parentheses
around parts of the expression
>>> (212 - 32.0) * 5.0 / 9.0
100.0

Variables and the Assignment Statement
• Variable is just a name that has a value associated with it.
• Variables’ names can use letters, digits, and the underscore
symbol.
• For example, X, species5618, and degrees_celsius are all allowed,
but 777 isn’t.
• You create a new variable simply by giving it a value:
>>> degrees_celsius = 26.0
• This statement is called an assignment statement; we say that
degrees_celsius is assigned the value 26.0.
• An assignment statement is executed as follows:
1. Evaluate the expression on the right of the = sign.
2. Store that value with the variable on the left of the = sign.

• In the diagram below, we can see the memory model for the result
of the assignment statement.
• Once a variable has been created, we can use its value in other
calculations.
>>> 100 - degrees_celsius
74.0.
• Whenever the variable’s name is used in an expression, Python
uses the variable’s value in the calculation.
•This means that we can create new variables from old ones:
>>> difference = 100 - degrees_celsius
• Typing in the name of a variable on its own makes Python display
its value:
>>> difference
74.0

• Variables are called variables because their values can change as
the program executes.
>>> difference = 100 - 15.5
>>> difference
84.5
• This does not change the results of any calculations done with that
variable before its value was changed:
>>> difference = 20
>>> double = 2 * difference
>>> double
40
>>> difference = 5
>>> double
40

• We can even use a variable on both sides of an assignment
statement:
>>> number = 3
>>> number
3
>>> number = 2 * number
>>> number
6
>>> number = number * number
>>> number
36
• When a statement like number = 2 * number is evaluated, Python
does the following:
1. Gets the value currently associated with number
2. Multiplies it by 2 to create a new value
3. Assigns that value to number

Combined Operators
• In the previous example, the variable number appeared on both
sides of the assignment statement.
• This is so common that Python provides a shorthand notation for
this operation:
>>> number = 100
>>> number -= 80
>>> number
20
• Here is how a combined operator is evaluated:
1. Evaluate the expression to the right of the = sign.
2. Apply the operator attached to the = sign to the variable
and the result of the expression.
3. Assign the result to the variable to the left of the = sign.

How Python Tells You Something Went Wrong
• There are two kinds of errors in Python:
• Syntax errors : happens when you type something that isn’t
valid Python code.
• Semantic errors : happens when you tell Python to do
something that it just can’t do, like divide a number by zero or try
to use a variable that doesn’t exist.
• Here is what happens when we try to use a variable that hasn’t
been created yet:
>>> 3 + moogah
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'moogah' is not defined

• The first two lines aren’t much use right now, though they’ll be
indispensable when we start writing longer programs.
• The last line is the one that tells us what went wrong: the name
moogah wasn’t recognized.
• The rules governing what is and isn’t legal in a programming
language are called its syntax.
• The message tells us that we violated Python’s syntax rules.

A Single Statement That Spans Multiple Lines
• In order to split up a statement into more than one line, you need
to do one of two things:
1. Make sure your line break occurs inside parentheses, or
2. Use the line-continuation character, which is a backslash, .
• Note that the line-continuation character is a backslash (), not the
division symbol (/).
• Here are examples of both:
>>> (2 +
... 3)
5
>>> 2 +
... 3
5

• Notice how we don’t get a SyntaxError.
• Each triple-dot prompt in our examples indicates that we are in the
middle of entering an expression; we use them to make the code
line up nicely.
• Here is one more example: let’s say we’re baking cookies.
• The authors live in Canada, which uses Celsius, but we own
cookbooks that use Fahrenheit.
• So how long it will take to preheat our oven? Here are our facts:
• The room temperature is 20 degrees Celsius.
• Our oven controls use Celsius, and the oven heats up at 20
degrees per minute.
• Our cookbook uses Fahrenheit, and it says to preheat the
oven to 350 degrees.

• We can convert t degrees Fahrenheit to t degrees Celsius like this:
(t - 32) * 5 / 9.
• Let’s use this information to try to solve our problem.
>>> room_temperature_c = 20
>>> cooking_temperature_f = 350
>>> oven_heating_rate_c = 20
>>> oven_heating_time = (
... ((cooking_temperature_f - 32) * 5 / 9) - room_temperature_c) /
... oven_heating_rate_c
>>> oven_heating_time
7.833333333333333

Designing and Using Functions
• Mathematicians create functions to make calculations easy to
reuse and to make other calculations easier to read because they
can use those functions instead of repeatedly writing
out equations.
• Programmers do this too.

Functions That Python Provides
• Python comes with many built-in functions that perform common
operations.
• One example is abs, which produces the absolute value of a
number:
>>> abs(-9)
9
>>> abs(3.3)
3.3
• Each of these statements is a function call.
• The general form of a function call is as follows:
«function_name»(«arguments»)

• Here are the rules to executing a function call:
1. Evaluate each argument one at a time, working from left
to right.
2. Pass the resulting values into the function.
3. Execute the function. When the function call finishes, it
produces a value.
• As function calls produce values, they can be used in expressions:
>>> abs(-7) + abs(3.3)
10.3
• We can also use function calls as arguments to other functions:
>>> pow(abs(-2), round(4.3))
16

>>> pow(abs(-2), round(4.3))
16
• Python sees the call on pow and starts by evaluating the arguments
from left to right.
• The first argument is a call on function abs, so Python executes it.
• abs(-2) produces 2, so that’s the first value for the call on pow.
• Then Python executes round(4.3), which produces 4.

• Some of the most useful built-in functions are ones that convert
from one type to another.
>>> int(34.6)
34
>>> int(-4.3)
-4
>>> float(21)
21.0
• In this example, we see that when a floating-point number is
converted to an integer, it is truncated, not rounded.
• If you’re not sure what a function does, try calling built-in function
help, which shows documentation for any function:
>>> help(abs)
Help on built-in function abs in module builtins:
abs(...)
abs(number) -> number
Return the absolute value of the argument.

• Another built-in function is round, which rounds a floating-point
number to the nearest integer:
>>> round(3.8)
4
>>> round(3.3)
3
>>> round(3.5)
4
>>> round(-3.3)
-3
>>> round(-3.5)
-4

• Function round can be called with one or two arguments.
• If called with one, it rounds to the nearest integer.
•If called with two, it rounds to a floating-point number, where the
second argument indicates the precision:
>>> round(3.141592653, 2)
3.14
• Let’s explore built-in function pow by starting with its help
documentation:
>>> help(pow)
Help on built-in function pow in module builtins:
pow(...)
pow(x, y[, z]) -> number
With two arguments, equivalent to x**y. With three
arguments, equivalent to (x**y) % z, but may be more
efficient (e.g. for longs).

• This shows that function pow can be called with either two or
three arguments.
• The English description mentions that when called with two
arguments it is equivalent to x ** y.
>>> pow(2, 4)
16
• This call calculates 24.
• How about with three arguments?
>>> pow(2, 4, 3)
1
• We know that 24 is 16, and evaluation of 16 % 3 produces 1.

Tracing Function Calls in the Memory Model
>>> def f(x):
... x = 2 * x
... return x
...
>>> x = 1
>>> x = f(x + 1) + f(x + 2)
• Are all the x’s the same variable?
• Does Python make a new x for each assignment? For each
function call?
• For each function definition?

• Here’s the answer: whenever Python executes a function call, it
creates a namespace (literally, a space for names) in which to store
local variables for that call.
• You can think of a namespace as a scrap piece of paper: Python
writes down the local variables on that piece of paper, keeps track
of them as long as the function is being executed, and throws that
paper away when the function returns.
• Separately, Python keeps another namespace for variables created
in the shell.
• Means the x that is a parameter of function f is a different variable
than the x in the shell.

• Let’s refine our rules for executing a function call to include this
namespace creation:
1. Evaluate the arguments left to right.
2. Create a namespace to hold the function call’s local variables,
including the parameters.
3. Pass the resulting argument values into the function by
assigning them to the parameters.
4. Execute the function body.

• In our memory model, we will draw a separate box for each
namespace to indicate that the variables inside it are in a separate
area of computer memory.
• The programming world calls this box a frame.
• We separate the frames from the objects by a vertical dotted line:
• Let’s trace that confusing code. At the beginning, no variables
have been created; Python is about to execute the function
definition.

•We have indicated this with an arrow:
➤ >>> def f(x):
... x = 2 * x
... return x
...
>>> x = 1
>>> x = f(x + 1) + f(x + 2)
• When Python executes that function definition, it creates a
variable f in the frame for the shell’s namespace plus a function
object.

• Now we are about to execute the first assignment to x in the shell.
>>> def f(x):
... x = 2 * x
... return x
...
➤ >>> x = 1
>>> x = f(x + 1) + f(x + 2)
• Once that assignment happens, both f and x are in the frame for
the shell.
• Now we are about to execute the second assignment to x in the
shell:
>>> def f(x):
... x = 2 * x
...
>>> x = 1
>>> x = f(x + 1) + f(x + 2)

• We first evaluate the expression on the right of the =, which is f(x
+ 1) + f(x + 2).
• Python evaluates the left function call first: f(x + 1).
• Python evaluates the argument, x + 1.
• In order to find the value for x, Python looks in the current frame.
• The current frame is the frame for the shell, and its variable x refers
to 1, so x + 1 evaluates to 2.
• After evaluating the argument to f, the next step is to create a
namespace for the function call.

• We draw a frame, write in parameter x, and assign 2 to that
parameter:
• There are two variables called x, and they refer to different values.
• Python will always look in the current frame, which we will draw
with a thicker border.

• We are now about to execute the first statement of function f:
>>> def f(x):
➤ x = 2 * x
... return x
...
>>> x = 1
>>> x = f(x + 1) + f(x + 2)
• x = 2 * x is an assignment statement.
• The right side is the expression 2 * x.
• Python looks up the value of x in the current frame and finds 2, so
that expression evaluates to 4.
• Python finishes executing that assignment statement by making x
refer to that 4.

• We are now about to execute the second statement of function f:
>>> def f(x):
... x = 2 * x
➤ ... return x
...
>>> x = 1
>>> x = f(x + 1) + f(x + 2)
• This is a return statement, so we evaluate the expression, which is
simply x.
• Python looks up the value for x in the current frame and finds 4, so
that is the return value.
• When the function returns, Python comes back to this expression:
f(x + 1) + f(x + 2).
• Python just finished executing f(x + 1), which produced the value 4.
• It then executes the right function call: f(x + 2).

• Following the rules for executing a function call, Python evaluates
the argument, x + 2.
• In order to find the value for x, Python looks in the current frame.
• The call on function f has returned, so that frame is erased: the
only frame left is the frame for the shell, and its variable x still
refers to 1, so x + 2 evaluates to 3.
• Now we have evaluated the argument to f. The next step is to
create a namespace for the function call.
• We draw a frame, write in the parameter x, and assign 3 to that
parameter:

We are now about to execute the first statement of function f:
>>> def f(x):
➤ x = 2 * x
... return x
...
>>> x = 1
>>> x = f(x + 1) + f(x + 2)
• x = 2 * x is an assignment statement.
•The right side is the expression 2 * x.
• Python looks up the value of x in the current frame and finds 3, so
that expression evaluates to 6.
• Python finished executing that assignment statement by making x
refer to that 6.

• We are now about to execute the second statement of function f:
>>> def f(x):
...
...
x = 2 * x
➤ return x
...
>>> x = 1
>>> x = f(x + 1) + f(x + 2)
• This is a return statement, so we evaluate the expression, which is
simply x.
• Python looks up the value for x in the current frame and finds 6, so
that is the return value.

• When the function returns, Python comes back to this expression:
f(x + 1) + f(x + 2).
• Python just finished executing f(x + 2), which produced the value 6.
• Both function calls have been executed, so Python applies the +
operator to 4 and 6, giving us 10.
• We have now evaluated the right side of the assignment statement;
Python completes it by making the variable on the left side, x, refer
to 10.

Omitting a Return Statement: None
• If you don’t have a return statement in a function, nothing is
produced:
>>> def f(x):
... x = 2 * x
...
>>> res = f(3)
>>> res
>>>
• That can’t be right—if res doesn’t have a value, shouldn’t we get a
NameError? Let’s poke a little more:
>>> print(res)
None
>>> id(res)
1756120

• Variable res has a value: it’s None! And None has a memory address.
• If you don’t have a return statement in your function, your function
will return None.
• You can return None yourself if you like:
>>> def f(x):
... x = 2 * x
... return None
...
>>> print(f(3))
None
• The value None is used to signal the absence of a value.

Working with Text
Creating Strings of Characters
• In Python, text is represented as a string, which is a sequence of
characters.
• We indicate that a value is a string by putting either single or
double quotes around it.
• Single and double quotes are equivalent except for strings that
contain quotes.
• Here are two examples: The opening and closing quotes
must match:
>>> 'Aristotle‘ >>> 'Charles Darwin"
'Aristotle‘ File "<stdin>", line 1
>>> "Isaac Newton“ 'Charles Darwin"
'Isaac Newton’ ^
SyntaxError: EOL while scanning string literal

• Strings can contain any number of characters, limited only by
computer memory.
• The shortest string is the empty string, containing no characters at
all:
>>> ‘ '
‘ '
>>> “"
‘ '

Operations on Strings
• Python has a built-in function, len, that returns the number of
characters between the opening and closing quotes:
>>> len('Albert Einstein')
15
>>> len('123!')
4
>>> len(' ')
1
>>> len('')
0
• We can add two strings using the + operator, which produces a new
string containing the same characters as in the two operands:
>>> 'Albert' + ' Einstein'
'Albert Einstein‘
• When + has two string operands, it is referred to as the
concatenation operator.

• Adding an empty string to another string produces a new string
that is just like the nonempty operand:
>>> "Alan Turing" + ''
'Alan Turing'
>>> "" + 'Grace Hopper'
'Grace Hopper‘
• Can operator + be applied to a string and a numeric value? If so,
would addition or concatenation occur?
>>> 'NH' + 3
TypeError: Can't convert 'int' object to str implicitly
• Here, Python took exception to our attempts to combine values of
different data types because it didn’t know which version of + we
want: the one that adds numbers or the one that concatenates
strings.

>>> 9 + ' planets'
TypeError: unsupported operand type(s) for +: 'int' and 'str‘
• Here, because Python saw a 9 first, it expected the second operand
to also be numeric. The order of the operands affects the error
message.
• The concatenation operator must be applied to two strings.
• If you want to join a string with a number, function str can be
applied to the number to get a string representation of it, and then
the concatenation can be done:
>>> 'Four score and ' + str(7) + ' years ago'
'Four score and 7 years ago'

• Function int can be applied to a string whose contents look like an
integer, and float can be applied to a string whose contents are
numeric:
>>> int('0')
0
>>> int("11")
11
>>> int('-324')
-324
>>> float('-324')
-324.0
>>> float("56.34")
56.34
>>> int('a')
ValueError: invalid literal for int() with base 10: 'a'
>>> float('b')
ValueError: could not convert string to float: 'b'

• In addition to +, len, int, and float, operator * can be applied to
strings.
• A string can be repeated using operator * and an integer, like this:
>>> 'AT' * 5
'ATATATATAT'
>>> 4 * '-'
'----‘
• If the integer is less than or equal to zero, the operator yields the
empty string (a string containing no characters):
>>> 'GC' * 0
''
>>> 'TATATATA' * -3
''

• Strings are values, so you can assign a string to a variable.
• Also, operations on strings can be applied to those variables:
>>> sequence = 'ATTGTCCCCC'
>>> len(sequence)
10
>>> new_sequence = sequence + 'GGCCTCCTGC'
>>> new_sequence
'ATTGTCCCCCGGCCTCCTGC'
>>> new_sequence * 2
'ATTGTCCCCCGGCCTCCTGCATTGTCCCCCGGCCTCCTGC'

Using Special Characters in Strings
• Suppose you want to put a single quote inside a string. If you write
it directly, an error occurs:
>>> 'that's not going to work'
File "<stdin>", line 1
'that's not going to work'
^
SyntaxError: invalid syntax
• When Python encounters the second quote—the one that is
intended to be part of the string—it thinks the string is ended.
• Then it doesn’t know what to do with the text that comes after the
second quote.

• One simple way to fix this is to use double quotes around the string.
>>> "that's better"
"that's better"
>>> 'She said, "That is better."'
'She said, "That is better."‘
• If you need to put a double quote in a string, you can use single
quotes around the string.
• But what if you want to put both kinds of quote in one string? You
could do this:
>>> 'She said, "That' + "'" + 's hard to read."'
'She said, "That's hard to read."‘
• The backslash is called an escape character, and the combination of
the backslash and the single quote is called an escape sequence.

• The escape sequence ' is indicated using two symbols, but those
two symbols represent a single character.
• The length of an escape sequence is one:
>>> len(''')
1
>>> len('it's')
4
• Here are some common escape sequences:

Creating a Multiline String
• If you create a string using single or double quotes, the whole string
must fit onto a single line.
• Here’s what happens when you try to stretch a string across
multiple lines:
>>> 'one
File "<stdin>", line 1
'one
^
SyntaxError: EOL while scanning string literal
• To span multiple lines, put three single quotes or three double
quotes around the string instead of one of each.
>>> '''one
... two
... three'''
'onentwonthree’

Printing Information
• We will use print to print messages to the users of our program.
• Those messages may include the values that expressions produce
and the values that the variables refer to.
>>> print(1 + 1)
2
>>> print("The Latin 'Oryctolagus cuniculus' means 'domestic
rabbit'.")
The Latin 'Oryctolagus cuniculus' means 'domestic rabbit'.
• Function print doesn’t allow any styling of the output: no colors, no
italics, no boldface. All output is plain text.
• The second ex. strips off the quotes around the string and shows us
the string’s contents rather than its representation.

>>> print('onettwonthreetfour')
one two
three four
• The example above shows how the tab character t can be used to
lay values out in columns.
• When a multiline string is printed, those n sequences are displayed
as new lines:
>>> numbers = '''one
... two
... three'''
>>> numbers
'onentwonthree'
>>> print(numbers)
one
two
three

• Function print takes a comma-separated list of values to print and
prints the values with a single space between them and a newline
after the last value:
>>> print(1, 2, 3)
1 2 3
>>>
• When called with no arguments, print ends the current line,
advancing to the next one:
>>> print()
>>>
• Function print can print values of any type, and it can even print
values of different types in the same function call:
>>> print(1, 'two', 'three', 4.0)
1 two three 4.0

• We separate each value with a comma and a space instead of just a
space by including sep=', ' as an argument:
>>> print('a', 'b', 'c') # The separator is a space by default
a b c
>>> print('a', 'b', 'c', sep=', ')
a, b, c
• Use the keyword argument end='' to tell Python to end with an
empty string instead of a new line:
>>> print('a', 'b', 'c', sep=', ', end='')
a, b, c>>>
• end='' is used only in programs, not in the shell.

Getting Information from the Keyboard
• Another built-in function that you will find useful is input, which
reads a single line of text from the keyboard.
• It returns whatever the user enters as a string, even if it looks
like a number:
>>> species = input()
Homo sapiens
>>> species
'Homo sapiens'
>>> population = input()
6973738433
>>> population
'6973738433'
>>> type(population)
<class 'str'>

• If you are expecting the user to enter a number, you must use int
or float to get an integer or a floating-point representation of the
string:
>>> population = input()
6973738433
>>> population
'6973738433'
>>> population = int(population)
>>> population
6973738433
>>> population = population + 1
>>> population
6973738434

• This time function int is called on the result of the call to input.
>>> population = int(input())
6973738433
>>> population = population + 1
6973738434
• Input can be given a string argument, which is used to prompt the
user for input (notice the space at the end of our prompt):
>>> species = input("Please enter a species: ")
Please enter a species: Python curtus
>>> print(species)
Python curtus

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