Python

1. Python#

I am going to summarize a few important ideas but this will not be an comprehensive introduction to Python. If you want that, please look to the guides at Real Python or the textbook Think Python by Downey.

1.1. Basics#

1.1.1. Installation#

I recommend following the instructions at https://docs.anaconda.com/miniconda/install/ to get miniconda. Alternatively, you can use the full anaconda distribution, manage your own environments and dependancies with pip/pyenv, or you can use parts of this website via binder.

1.1.2. Hello World#

print("Hello, World!")

1.1.3. Variable assignment#

x = 1
y = 4
z = x

print(x, z)
z = 5
print(x, z)

1.1.4. Comments#

Comments are lines that start with #. There are no block comments like there are in C. However, there are multiline strings that look like block comments.

If you make a python string, but do not assign it to a variable, then it acts like similar to a block comment.

"""This is a multiline string!
It is really helpful for documenting functions.
"""

1.1.5. Types#

  • Text Type: str

  • Numeric Types: int, float, complex

  • Sequence Types: list, tuple, range

  • Mapping Type: dict

  • Set Types: set, frozenset

  • Boolean Type: bool

  • Binary Types: bytes, bytearray, memoryview

  • None Type: NoneType

print(type(var))

>>> x = [1,2,3]
>>> z = x
>>> z[1]=5
>>> x
[1, 5, 3]

Note

However, Python is “duck” typed: if it walks like a duck and talks like a duck, it is a duck. This loose typing can confuse some people, but you will find out that it can be really helpful for types to build off each other and fall back to a more basic type via duck typing rather than a lower level of code needing to know about all extensions to lists.

1.1.6. if … else#

a = 200
b = 33
if b > a:
  print("b is greater than a")
elif a == b:
  print("a and b are equal")
else:
  print("a is greater than b")

1.1.6.1. Keywords#

There are useful keywords like the boolean operators and, or, and not. Additionally it is best practice to use the the comparison operator is rather than checking for equality. This is especially true for to check if a variable is None. Finally, the keyword pass is sometimes used.

For more, check out w3schools or Read Python.

1.1.7. for loops#

Python has a for loop and a while loop, but typically we use for-in loops. The syntax is as follows:

for item in list:
  print(item)

for i in range(5):
  print(i)

For loops, check out the keywords break and continue.

You can even use Python to write C. Please don’t.

a = ['Mary', 'had', 'a', 'little', 'lamb']
for i in range(len(a)):
    print(i, a[i])

In this case, we used the built in function len to get the length of an object. Check what types work with len?

Note

If you are using a for loop—or its variants—in python, you are likely doing something wrong. It may be someone else’s fault, but something is not right.

For more advanced uses, you may want to use the match keyword.

1.1.8. Functions & Classes#

If there is a bit of code you want to reuse, like calculating the a chi-square you want to minimize, the standard practice is to put it into a function.

Important

If you are copying and pasting while writing your code, you are likely doing something wrong and you should probabliy make a function or a class.

Functions are defined with the def keyword as seen here:

def function_name(var1, var2):
  return var1*var2

As you advance in your Python skills, you will learn how to take this simple function and adorn it with additional useful features such as docstrings, keyword only arguments, and decorators. We will address each of these as needed later in the course.

Python is an object oriented language, but we will mostly use its functional/scripting language features. So know that there are classes but we will not use them much.

1.2. More Advanced Python#

1.2.1. String formating#

If you have a variable that you want to print with a specific formatting, like a float that you want to report with the correct significant digits, you will use string formatting and f-strings. Check out w3school’s and real python’s tutorials on this topic.

1.2.2. Modules#

import this

1.2.3. Scope#

x = 300

def myfunc():
  x = 200
  print(x)

myfunc()

print(x)

def myfunc():
  global x
  x = 300

myfunc()

print(x)

1.2.4. try except finally#

We will occasionally use try blocks to deal with errors. You can read up on them in detail at w3school.

1.2.5. Reading files#

1.3. Suggested Reading#

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