Python 1: print, variables, strings & f-strings

What you will learn. This module teaches the very first building blocks of Python, one small idea per chapter: showing text with print, storing values in variables, the difference between text and numbers, cleaning text with string methods, building configuration lines with f-strings, yes/no values called booleans, and how to read and fix the three errors every beginner meets. You do not need any programming background. If you finished the module "What is network automation?", you have already run a script; here you learn to write the lines yourself.

Prerequisites. Basic networking: interface names such as GigabitEthernet0/1, VLAN numbers and IP addresses. Everything in this module runs on the lab host AUTO; no routers are needed.

Analogy: labelled jars and a mail-merge letter

Picture a kitchen shelf with jars. Each jar has a label (salt, sugar) and something inside. A variable is a jar: a name on the outside, a value inside. Now picture a printed letter that says "Dear ____, your VLAN is ____". A clerk fills the blanks from a list. That is an f-string: a sentence with blanks that Python fills from your jars. Together they let one small script produce the exact configuration for any port, any VLAN, any site.

A first look at the lab script

In Lab 1 you get hello_router.py. Read it now even if you do not understand every word yet; by chapter 6 you will understand all of it. Lines starting with # are comments.

# hello_router.py - build interface config lines with Python
# A VARIABLE is a named box that stores a value.
interface = "GigabitEthernet0/1"   # a string (text): always inside quotes
vlan = 10                          # an integer (whole number): no quotes
site = "  pune  "                  # messy text with extra spaces

# print() shows something on the screen
print("Building config...")

# .strip() removes spaces at both ends, .upper() makes CAPITAL letters
clean_site = site.strip().upper()

# f-string: put f before the quote, then {name} is replaced by its value
print(f"interface {interface}")
print(f" description Link to {clean_site} - VLAN {vlan}")
print(" switchport mode access")
print(f" switchport access vlan {vlan}")

AUTO> python3 hello_router.py
Building config...
interface GigabitEthernet0/1
 description Link to PUNE - VLAN 10
 switchport mode access
 switchport access vlan 10

Five short lines of output came from seventeen lines of script. Change one variable, say vlan = 20, and the text changes with it. You never retype the configuration.

The eight chapters ahead

1. print and comments
2. Variables
3. Strings, integers and types
4. String methods
5. f-strings
6. Numbers, maths and booleans
7. Fixing errors (the report lab)
8. Summary and checklist

Every script needs a way to show something to you. In Python that is print(). It is like the show command: it displays information on the screen so that you can read it. This chapter teaches print and the comment, the two simplest tools you will use in every script.

print() shows what is in the brackets

print("Hello, router")
print("VLAN", 10)
print("Gi0/1", "is", "up")
print()
print("end")

AUTO> python3 p1.py
Hello, router
VLAN 10
Gi0/1 is up

end

• Text goes inside quotes. Single or double quotes both work. The quotes are not printed.
• A number needs no quotes: 10.
• Several things separated by commas are printed with one space between them.
• print() with nothing inside prints an empty line, handy to space out a report.

Quotes inside text

If your text contains a quote mark, choose the other kind around it. Python also understands special characters written with a backslash: \n means "new line" and \t means "tab".

print("It's up")
print('Description "Core link"')
print("line one\nline two")
print("a\tb")

AUTO> python3 p2.py
It's up
Description "Core link"
line one
line two
a	b

Two options of print are worth knowing. sep changes the separator between items and end changes what is printed at the very end (normally a new line):

print("a", "b", "c", sep="-")
print("no newline", end="")
print(" <- same line")

AUTO> python3 p3.py
a-b-c
no newline <- same line

Comments: notes for humans

A comment starts with #. Python ignores the rest of that line. Use comments to say why a line exists. A comment can sit on its own line or at the end of a code line. Good scripts are readable months later because of comments.

# This whole line is a comment: Python skips it.
print("Report start")   # a comment at the end of a code line
# print("this line is switched off")
print("Report end")

AUTO> python3 p4.py
Report start
Report end

The third line shows a useful trick: putting # in front of a line "switches it off" without deleting it. Programmers call this commenting out.

The classic first error

If you open a quote and forget to close it, Python cannot read the file at all. It reports a SyntaxError before running anything:

print("Hello)

AUTO> python3 p5.py
  File "p5.py", line 1
    print("Hello)
          ^
SyntaxError: unterminated string literal (detected at line 1)

Read it from the bottom: SyntaxError: unterminated string literal means a piece of text has no closing quote. The small ^ arrow points at where Python noticed the problem.

Imagine you are configuring ten ports for VLAN 10 and your manager changes the VLAN to 20. If you typed "10" in ten places you must find and change all ten. If you typed it once, in a labelled place, you change one thing. A variable is that labelled place: a name that points to a value. Variables are how scripts remember things.

Creating a variable

You create a variable with a single equals sign. Read = as "gets" or "is set to", not as "equals": the name on the left receives the value on the right.

hostname = "R1"
vlan = 10
site = "PUNE"
print(hostname, vlan, site)

AUTO> python3 v1.py
R1 10 PUNE

You can change what is in a box at any time by assigning again. Python always works out the right side first, then stores it. So vlan = vlan + 1 means "take the value in vlan, add 1, put the answer back in vlan".

vlan = 10
print("before:", vlan)
vlan = 20
print("after:", vlan)
vlan = vlan + 1
print("plus one:", vlan)
backup = vlan
vlan = 99
print(vlan, backup)

AUTO> python3 v2.py
before: 10
after: 20
plus one: 21
99 21

The last two lines show that backup = vlan copies the value at that moment. Changing vlan later does not change backup.

Rules for names

• Use letters, digits and underscores. The name cannot start with a digit.
• Names are case-sensitive: hostname and Hostname are different boxes.
• No spaces. Programmers write mgmt_ip (words joined by underscores, called snake_case).
• Choose meaningful names: mgmt_ip beats x. Names written in CAPITALS, such as ROUTERS, are a habit for settings that you do not intend to change while the script runs.

Two mistakes that beginners meet within the first hour. A name starting with a digit, and a change of case:

1ip = "10.99.0.1"

AUTO> python3 v3.py
  File "v3.py", line 1
    1ip = "10.99.0.1"
    ^
SyntaxError: invalid decimal literal

hostname = "R1"
print(Hostname)

AUTO> python3 v4.py
Traceback (most recent call last):
  File "v4.py", line 2, in <module>
    print(Hostname)
NameError: name 'Hostname' is not defined. Did you mean: 'hostname'?

NameError means Python was asked for a name that does not exist yet. Notice that Python even suggests the name you probably meant: Did you mean: 'hostname'?.

A phone book lists "Asha 9876" and a calculator adds 9876 + 1. Both show digits, but you would never add one to a phone number. Python draws the same line. Every value has a type, and the type decides what you are allowed to do with it. Two types do most of the work in network scripts: str (text) and int (whole numbers).

Type	Meaning	Examples
str	Text, always in quotes	"R1", "10.99.0.1", "Gi0/1"
int	Whole number, no quotes	10, 4094, -1
float	Number with a decimal point	2.5, 99.9
bool	Yes or no	True, False

The built-in type() tells you the type of anything. Notice that an IP address in quotes is text, not a number, and that "10" in quotes is also text:

print(type("10.99.0.1"))
print(type("10"))
print(type(10))
print(type(2.5))
print(type(True))

AUTO> python3 s1.py
<class 'str'>
<class 'str'>
<class 'int'>
<class 'float'>
<class 'bool'>

What + does depends on the type

With numbers, + adds. With strings, + joins them end to end. The * sign repeats a string, which is perfect for drawing a line in a report.

print(10 + 5)
print("10" + "5")
print("Gi" + "0/1")
print("-" * 20)
print("ab" * 3)

AUTO> python3 s2.py
15
105
Gi0/1
--------------------
ababab

"10" + "5" gave 105 (text joined), not 15. That is the whole difference between str and int in one line.

Converting between types

Output from routers always arrives as text. To do maths on a number that arrived as text, convert it with int(). To put a number inside text, convert it with str().

vlan_text = "10"
print(int(vlan_text) + 5)
vlan = 10
print("VLAN " + str(vlan))
print(float("2.5") + 1)

AUTO> python3 s3.py
15
VLAN 10
3.5

Mixing the two types without converting is an error. Python refuses to guess:

vlan = 10
print("VLAN " + vlan)

AUTO> python3 s4.py
Traceback (most recent call last):
  File "s4.py", line 2, in <module>
    print("VLAN " + vlan)
TypeError: can only concatenate str (not "int") to str

TypeError: can only concatenate str (not "int") to str says: you tried to join text with a number. Fix it with str(vlan), or, even better, with an f-string (chapter 6).

Looking inside a string

A string is a row of characters. len() counts them. A number in square brackets picks one character, counting from 0; a negative number counts from the end; two numbers with a colon take a slice (the end position is not included).

name = "PUN-CORE-01"
print(len(name))
print(name[0])
print(name[-1])
print(name[0:3])

AUTO> python3 s5.py
11
P
1
PUN

Text from the real world is messy. A site name arrives as " pune " with extra spaces and small letters, an interface as "GigabitEthernet0/1" when a report wants "Gi0/1". Python strings come with built-in actions called methods for exactly these jobs. A method is called with a dot after the value: value.method().

Cleaning methods

site = "  pune  "
print("[" + site + "]")
print("[" + site.strip() + "]")
print(site.strip().upper())
print("R1-CORE".lower())
print("pune".title())

AUTO> python3 m1.py
[  pune  ]
[pune]
PUNE
r1-core
Pune

• strip() removes spaces (and new lines) from both ends.
• upper(), lower(), title() change the case.
• You can chain methods: site.strip().upper() strips first, then capitalises.

Important: a method never changes the original. It returns a new string. If you want to keep the result, store it in a variable:

site = "  pune  "
site.strip()
print("[" + site + "]")
site = site.strip()
print("[" + site + "]")

AUTO> python3 m2.py
[  pune  ]
[pune]

The first strip() threw its answer away, so site still has spaces. The assignment site = site.strip() keeps it.

Replacing and checking

intf = "GigabitEthernet0/1"
print(intf.replace("GigabitEthernet", "Gi"))
print(intf.startswith("Gig"))
print(intf.startswith("Fast"))
print("0/1" in intf)
print(intf.endswith("1"))
print("a-b-c".count("-"))

AUTO> python3 m3.py
Gi0/1
True
False
True
True
2

replace(old, new) swaps every copy of old for new. startswith, endswith and the word in answer yes/no questions with True or False. They are the building blocks of the decisions you will write in the next module.

Cutting and joining: split and join

split() cuts a string into a list of pieces. With no argument it cuts at any run of spaces; with an argument it cuts at that character. join() does the opposite: the separator comes first, then the list.

line = "Gi0/0 10.99.0.1 up up"
parts = line.split()
print(parts)
print(parts[1])
print("10.99.0.1".split("."))
print("PUN-CORE-01".split("-")[0])
print(",".join(["Gi0/1", "Gi0/2", "Gi0/3"]))

AUTO> python3 m4.py
['Gi0/0', '10.99.0.1', 'up', 'up']
10.99.0.1
['10', '99', '0', '1']
PUN
Gi0/1,Gi0/2,Gi0/3

The line above looks like a row from show ip interface brief. After split() each column is one item of the list, and parts[1] is the IP address. This is exactly how scripts read device output.

Method	Result for " Gi0/1 "
strip()	"Gi0/1"
strip().upper()	"GI0/1"
strip().replace("Gi", "Fa")	"Fa0/1"
split("/")	[' Gi0', '1 ']

You can already join text with + and convert numbers with str(). It works, but it is clumsy: "interface " + name + " vlan " + str(vlan) is easy to mistype. Python has a much nicer tool, the f-string (formatted string literal). It is a template with blanks. Put the letter f directly before the opening quote, and write {name} wherever you want a variable's value to appear.

interface = "GigabitEthernet0/2"
vlan = 20
site = "PUNE"
print(f"interface {interface}")
print(f" description Link to {site} - VLAN {vlan}")
print(f" switchport access vlan {vlan}")
print(f" ip address 10.1.{vlan}.1 255.255.255.0")

AUTO> python3 f1.py
interface GigabitEthernet0/2
 description Link to PUNE - VLAN 20
 switchport access vlan 20
 ip address 10.1.20.1 255.255.255.0

The output is configuration that you can paste straight into a switch. Three details matter:

• The f must be directly before the quote, with no space.
• Numbers need no str(); the f-string converts them for you.
• Inside the braces you may put a calculation or a method: {site.lower()}, {vlan + 100}.

site = "PUNE"
vlan = 20
print(f"{site.lower()}-sw-{vlan + 100}")

AUTO> python3 f2.py
pune-sw-120

Formatting numbers

After the value you may add a colon and a format. {n:03d} pads a whole number with zeros to three digits; {x:.2f} shows two decimals; {text:>8} right-aligns text in 8 characters. Useful for neat, equal-width names like SW-007.

n = 7
print(f"SW-{n:03d}")
print(f"{3.14159:.2f}")
print(f"[{'Gi0/1':>8}]")

AUTO> python3 f3.py
SW-007
3.14
[   Gi0/1]

Two traps: the missing f and literal braces

Forget the f and Python prints the braces exactly as typed. If you want real braces in the output, double them:

vlan = 10
print("vlan {vlan}")
print(f"vlan {vlan}")
print(f"{{vlan}} is {vlan}")

AUTO> python3 f4.py
vlan {vlan}
vlan 10
{vlan} is 10

Lab 1 walk-through

In the lab you run hello_router.py, change two variables and add one line. Here are the lines with their numbers, so you know which line to edit:

AUTO> nl hello_router.py
     1  # hello_router.py - build interface config lines with Python
     2  # A VARIABLE is a named box that stores a value.
     3  interface = "GigabitEthernet0/1"   # a string (text): always inside quotes
     4  vlan = 10                          # an integer (whole number): no quotes
     5  site = "  pune  "                  # messy text with extra spaces
     6  
     7  # print() shows something on the screen
     8  print("Building config...")
     9  
    10  # .strip() removes spaces at both ends, .upper() makes CAPITAL letters
    11  clean_site = site.strip().upper()
    12  
    13  # f-string: put f before the quote, then {name} is replaced by its value
    14  print(f"interface {interface}")
    15  print(f" description Link to {clean_site} - VLAN {vlan}")
    16  print(" switchport mode access")
    17  print(f" switchport access vlan {vlan}")

First run with the original values, then change the port and the VLAN (lines 3 and 4 only), and run again:

AUTO> python3 hello_router.py
Building config...
interface GigabitEthernet0/1
 description Link to PUNE - VLAN 10
 switchport mode access
 switchport access vlan 10

AUTO> edit hello_router.py 3 interface = "GigabitEthernet0/2"
AUTO> sed -i 's/vlan = 10/vlan = 20/' hello_router.py

AUTO> python3 hello_router.py
Building config...
interface GigabitEthernet0/2
 description Link to PUNE - VLAN 20
 switchport mode access
 switchport access vlan 20

Finally add one more line at the end (the file has 17 lines, so insert as line 18) that prints the short interface name with replace():

AUTO> insert hello_router.py 18 print(interface.replace("GigabitEthernet", "Gi"))

AUTO> python3 hello_router.py
Building config...
interface GigabitEthernet0/2
 description Link to PUNE - VLAN 20
 switchport mode access
 switchport access vlan 20
Gi0/2

The site in the description is PUNE: the variable held " pune ", and .strip().upper() in line 11 cleaned it before the f-string used it. Line 4 holds vlan = 20 without quotes, so it is an int.

Networking is full of numbers: VLAN IDs, hop counts, uptime days, a /24 holding 254 hosts. Python can calculate all of them. It can also answer yes/no questions such as "is this VLAN in range?" The yes/no answers are called booleans. They are the doorway to decisions, which you will write in the next module.

The arithmetic operators

Operator	Meaning	Example
+ - *	add, subtract, multiply	10 * 3 is 30
/	divide (always gives a float)	7 / 2 is 3.5
//	whole-number divide	7 // 2 is 3
%	remainder	7 % 2 is 1
**	power	2 ** 8 is 256

print(10 * 3)
print(7 / 2)
print(7 // 2)
print(7 % 2)
print(2 ** 8)
print(2 ** 8 - 2)
print(int(7 / 2))
print(round(7 / 3, 2))

AUTO> python3 n1.py
30
3.5
3
1
256
254
3
2.33

2 ** 8 - 2 is 254: a /24 has 8 host bits, so 256 addresses, minus the network and broadcast addresses. Python follows normal maths order: power first, then multiply and divide, then add and subtract; use brackets to be sure.

A small surprise: computers store decimals approximately, so very small errors can appear.

print(0.1 + 0.2)
print(round(0.1 + 0.2, 2))
print(type(3 / 1))
print(3 + 2.5)

AUTO> python3 n2.py
0.30000000000000004
0.3
<class 'float'>
5.5

Booleans: True and False

A boolean has only two possible values, True and False (capital first letter, no quotes). You get them by comparing. A comparison asks a question and returns a boolean.

Comparison	Asks
a == b	Equal? (two equals signs)
a != b	Not equal?
a < b a > b	Smaller or bigger?
a <= b a >= b	Smaller or equal, bigger or equal?
x in text	Is x inside?

vlan = 10
uptime_days = 42
print(vlan == 10)
print(vlan != 10)
print(uptime_days > 30)
print(1 <= vlan <= 4094)
print("Gi" in "GigabitEthernet0/1")

AUTO> python3 n3.py
True
False
True
True
True

Booleans can be combined with and, or and not. and needs both sides true, or needs at least one, not flips the answer.

is_core = True
is_old = False
print(is_core and is_old)
print(is_core or is_old)
print(not is_old)
print(3 > 2 and 2 > 5)
print(bool(""), bool("R1"), bool(0), bool(5))

AUTO> python3 n4.py
False
True
True
False
False True False True

The last line shows "truthiness": an empty string and zero count as False; any non-empty text and any non-zero number count as True. Scripts use this to test "did the router return anything?".

Professionals do not write error-free code. They read errors well. Python reads the whole file first; if the text is not valid Python at all, it stops with a SyntaxError before anything runs. If the file is valid, Python runs line by line and stops at the first line that fails, printing a traceback. In both cases, read the last line first and then the line number. The loop is: run, read, fix one thing, run again.

Error	Means	Typical fix
SyntaxError	Text is not valid Python	Close the quote or bracket, add the colon
NameError	Unknown name	Fix the spelling or create the variable first
TypeError	Wrong type for the operation	Convert, or use an f-string

Lab 2: the report that never ran

A fresher wrote report.py to print a daily health report. It has three classic beginner bugs. Here is the file with line numbers:

AUTO> nl report.py
     1  # report.py - print a short health report for one router
     2  hostname = "BLR-EDGE-01"
     3  uptime_days = 42          # int (a number)
     4  interfaces_up = 3         # int (a number)
     5  
     6  print("=== Daily router report ===)
     7  print("Router: " + hostnme)
     8  print("Uptime in days: " + uptime_days)
     9  print(f"Interfaces up: {interfaces_up}")
    10  print("=== End of report ===")

Bug 1: SyntaxError

AUTO> python3 report.py
  File "report.py", line 6
    print("=== Daily router report ===)
          ^
SyntaxError: unterminated string literal (detected at line 6)

Python names the line (6) and the problem: a string with no closing quote. Notice that nothing printed at all, even though lines 1 to 5 look fine: the whole file is checked first. Fix line 6:

AUTO> edit report.py 6 print("=== Daily router report ===")

Bug 2: NameError

AUTO> python3 report.py
=== Daily router report ===
Traceback (most recent call last):
  File "report.py", line 7, in <module>
    print("Router: " + hostnme)
NameError: name 'hostnme' is not defined. Did you mean: 'hostname'?

Now the file is valid Python, so it starts running and prints the first line. It stops at line 7 because hostnme is a misspelling of hostname, and Python suggests the fix. Correct the spelling:

AUTO> sed -i 's/hostnme/hostname/' report.py

Bug 3: TypeError

AUTO> python3 report.py
=== Daily router report ===
Router: BLR-EDGE-01
Traceback (most recent call last):
  File "report.py", line 8, in <module>
    print("Uptime in days: " + uptime_days)
TypeError: can only concatenate str (not "int") to str

Line 8 tries to join the text "Uptime in days: " with the integer uptime_days. Use an f-string, as in chapter 6:

AUTO> edit report.py 8 print(f"Uptime in days: {uptime_days}")

AUTO> python3 report.py
=== Daily router report ===
Router: BLR-EDGE-01
Uptime in days: 42
Interfaces up: 3
=== End of report ===

The report is complete: the router name, 42 days of uptime and 3 interfaces up. Three errors, three small fixes, each found by reading one message.

You can now display text, store values, tell text from numbers, clean strings, build configuration with f-strings, compare values and read the three most common errors. These are the foundation of every automation script. This page is your revision sheet.

Can-do checklist

• Print text, numbers and several values with print().
• Write a comment and use it to switch a line off.
• Create and change variables; follow the naming rules.
• Say whether a value is str, int, float or bool, and convert with int() and str().
• Clean text with strip, upper, lower and replace; cut it with split.
• Build config lines with an f-string, including formats such as :03d.
• Use arithmetic and comparisons, and predict True or False.
• Fix a SyntaxError, a NameError and a TypeError from the message.

Mini glossary

Variable

A name bound to a value.

str / int / float / bool

Text / whole number / decimal / True-False.

Method

An action on a value: value.method().

f-string

A string starting with f whose braces are filled with values.

Comment

A note after #, ignored by Python.

Boolean

True or False, the result of a comparison.

Traceback

The report Python prints when a script fails.

Concatenate

Join two strings end to end.

Quick reference

Idea	Example	Result
Show text	print("R1", 10)	R1 10
Variable	vlan = 10	stores 10
Type	type("10")	<class 'str'>
Convert	int("10") + 5	15
Clean	" pune ".strip().upper()	PUNE
Cut	"a b".split()	['a', 'b']
f-string	f"vlan {vlan}"	vlan 10
Compare	10 == 10	True

Most tested facts

• Quotes make text: "10" + "5" is 105; 10 + 5 is 15.
• String methods return a new string; they do not change the original.
• Counting starts at 0; [0:3] excludes position 3.
• Without the f, an f-string is plain text and the braces print as typed.
• = stores, == compares; / always returns a float.
• SyntaxError appears before the script runs; NameError means an unknown name; TypeError means a wrong type.

Next: "Python 2" teaches lists, dictionaries, for loops and if/else decisions, so your scripts can handle many devices and choose what to do.