Python basics

cheatsheet.py

# variables hold values the user enters
# we also use them to guide program flow
# by the way - these are comments. They start with # - after that the rest of the line is ignored
number = 1 # define a variable
string = "Some text" # a string is a variable containing text
list = [1,2,3,4] # we can also define lists of things


number = 2  # changing the value of a variable
number = number + 1 # we can use the variable itself, any expression, to set the new value. The value of number is now 3
number = int("3")  # turning a string into a number
string = "The number is " + str(number) # turning a number into a string. The string is now "The number is 3"
number = list[0] # we can look up values in the list. The first value has index 0. The value of number is now 1
number = list[3] # this is the last value in the list. The value of number is now 4
# strings can be added
"This is now " + " a much longer string"

# useful functions
int(astring) # returns a number - if astring looks like a number. So "3", "34345" but not "three" or "Firstname Lastname".
str(anumber) # turns 3 into "3" and so on
print(anyvalue) # automatically convers anyvalue to string and prints it on the screen.

# program flow
# Using if statements we can do certain things only in certain cases
# In the following statement, we only print "The number is 1"
# when number has the value 1. Note the double == - that does not assign a value to number - but checks the value
if number == 1:
    print("The number is 1") #notice the indentation. In python indentation is important
print("I don't know what the number is") #this is not part of the if-statement, because it's not indented

# we can do a little more with if
if number == 1:
    print("The number is 1")
else:
    print("The number is not 1")

# We can keep doing something until a condition becomes False
# This bit of program prints
# 1
# 2
# .. up to 9
number = 1
while number < 10:
    print(number)
    number = number + 1

# Defining our own functions
# int, str and so on are nice - and there are hundreds of other functions
# but to build large programs we build our own functions that lets us do more complicated things
def afunction(aninput, anotherinput):
    avalue = aninput + anotherinput
    return avalue  # without return, the function has no output. That's totally ok - but might not be what you expect

thesum = afunction(2,3) #thesum is now 5

# You can put functions in other files
# in amodule.py
def anotherfunction():
  print("Imported this from amodule")


## and then here
import amodule
amodule.anotherfunction()
#
# OR
from amodule import anotherfunction
anotherfunction()

## Handling errors
try:
  int("Not a number")
except:
  print("That was not a number!")

int("Not a number") # the program stops, because we didn't contain the error

## saving ond loading things
# saving a number to a file. 
with open('filename', 'w') as f:
  f.write(str(number))

# reading it again
with open('filename') as f:
  number = int(f.read())