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Python Language Basics

Syntax

Blanks

code "print ()", then execute

Double quotes and single quotes

Double quotes equal to single quotes

[] does not work to specify precedence in formula

() is used to specify precedence in formula rather than []

For example, ((1+r)*n-1) can be executed, but [(1+r)*n-1] can not be executed.

[] have other function, like range\[1:50\]

Multiplication operator * can not be omitted in formula

Take an example:

when we exercise the mortgage calculator: A=P*r*(1+r)
is different from A=Pr\(1+r\), the latter one can not be executed.

Letter case difference

p = 1
print(P)

You will see the error message NameError: name 'P' is not defined . That is because Python is **case-sensitive. **The lowercase p and uppercase P mean different variables.

List range pattern

in range (a,b), it's close to the range in the left, open the range in the right.

For i in range(1,10):
    print (i)

You will see the outcome: 1,2,3,4,5,6,7,8,9 without 10, because the left range is included, while the right range is excluded.

== and =

In python, notation == represents making a judgment / comparison. Notation = means assign values.

For example:

df['location'] == '旺角'
#this means Iterate all elements in location column to compare whether its equal to 旺角
df['location'] = '旺角'
#this means assign 旺角 to every element in the column location

Append VS Extend

when a list of elements append to another list, the list passing into the function () will be treated as an whole element.

x = ['a','b']
y = [0,2,4,8]
x.append(y)
x

Output:

['a', 'b', [0, 2, 4, 8]]

extend method extract every single items of one list and add those items one by one into the new list.

x = ['a','b']
y = [0,2,4,8]
x.extend(y)
x

Output:

['a', 'b', 0, 2, 4, 8]

For loop range

Function 'Range' has 3 parameters. From XX to XX with the step size XX. The 3rd parameter is the step size. If you don't input the 3rd one, it will take 1 in default.

for i in range(5,10):
    print(i)
#5
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for i in range(5,10,2):
    print(i)
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The parameters can be negative. At the same time, you need adjust the positions of the first and second parameters.

for i in range(5,10,-1):
    print(i)
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Name clash with reserved word

We find the following error very common when students start to work on bigger projects, especially in Jupyter notebook.

TypeError: 'str' object is not callable

or

TypeError: 'list' object is not callable

In the first example, you are like to get following result when checking the type of str:

type(str)
# str

This is an indicator that str is reassigned to some other value before. Check if you have str=xxx statement earlier on. str is a built-in function. The variable assignment changes it to another object. That is why Python complains "'str' object is not callable".

The same issue goes with list. Students may use list = [] and then list.append(xxx) later to collect data entries. The name is intuitive but the assignment pollutes the built-in function.

Even if you change that line of assignment, Python still gives the same complaint. The reason is that Jupyter notebook has a continuously running Python engine (called "kernel") in the back. The assignment is already effective and will keep being effective in the whole session. You need to "restart kernerl" after fixing the code.

One needs to keep off reserved word like list, dict, str, etc. Except for that, you also want to keep away from the modules/ classes you import. One useful trick for beginner is to add my to your own variables when you are not sure if the variable name might clash with existing object or not.

Related issues: