Why different format method behave differently in Python?

Question

I was learning Python as a beginner. Recently I learnt about formatting methods, dictionaries and etc. Currently I was studying for loop and discovered a functioned called enumerate (probably nothing to do with is problem). I was applying what I had learnt till now by mixing everything. Suddenly I discovered that two format method acts differently!! How and why is this happening? Please explain.

Method 1:

nameAgeDictionary = {'Jack': 38, 'John': 51, 'Alex': 13, 'Alvin': 'Not Available'}

for index, name in enumerate(nameAgeDictionary):
    print('(%d) Name = %s, Age = %s' % (index+1, name, nameAgeDictionary[name]))  # Format_Method_1

Output:

(1) Name = Jack, Age = 38

(2) Name = John, Age = 51

(3) Name = Alex, Age = 13

(4) Name = Alvin, Age = Not Available

Method 2:

nameAgeDictionary = {'Jack': 38, 'John': 51, 'Alex': 13, 'Alvin': 'Not Available'}

for index, name in enumerate(nameAgeDictionary):
    print('({0:d}) Name = {1:s}, Age = {2:s}'.format(index+1, name, nameAgeDictionary[name])) # Format_Method_2

Output:

Traceback (most recent call last): File "PATH_to_File.py", line 3, in

print('({0:d}) Name = {1:s}, Age = {2:s}'.format(

ValueError: Unknown format code 's' for object of type 'int'

I have tried putting d in the place of s, on that case, it prints first 3 lines and gets stucked in last line (e.g. Not Available).

2.4.3. Formatted string literals. You can force the conversion to str with !. print('({0:d}) Name = {1:s}, Age = {2!s}'.format(index+1, name, nameAgeDictionary[name])) — Henry Ecker
– Henry Ecker ♦, Commented May 12, 2021 at 20:21
Thank you! That is working! But as assumed in the answer by user_na, that in contrast to .format() the % formatting calls str as a fallback. What's your opinion on that? I am asking this to you as well as user_na wasn't completely sure of the reason. — Pritam
– Pritam, Commented May 12, 2021 at 20:31

user_na · Accepted Answer · 2021-05-12 20:44:21Z

4

As the type of the age is mixed (strand int), just don't specify the type.

for index, name in enumerate(nameAgeDictionary):
    print('({0:d}) Name = {1:s}, Age = {2}'.format(index+1, name, nameAgeDictionary[name])) # Format_Method_2

By doing this __str__ of the input should be called which savely converts the int to str. The result is:

(1) Name = Jack, Age = 38
(2) Name = John, Age = 51
(3) Name = Alex, Age = 13
(4) Name = Alvin, Age = Not Available

I assume (but I am not completly sure) that in contrast to .format() the % formating calles __str__ as fallback.

Update

Here is the proof that % formating is calling __str__:

class test():
     def __str__(self):
         return 'bar'
 
foo = test()
print('%s'%(foo))

prints

bar

edited May 12, 2021 at 20:44

answered May 12, 2021 at 20:09

user_na

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4 Comments

Pritam Over a year ago

Thank you! This is working totally fine! But I still don't understand this thing: Why is in method 1 it is still working even after specifying the type as a string? (e.g. with %s)? Why is method 1 not showing the error that Method 2 was giving me?

user_na Over a year ago

As I said, I assume that the first one calls __str__ in case the input is not a string as specified in the input, while the second one does not handle the error.

Pritam Over a year ago

Oh! Accha, now I am starting to understand what might cause this behavior difference to happen. Thank you so much @user_na for elaborating this repeatedly :-)

Pritam Over a year ago

Wow, great! Thank you so much for your help @user_na, without your help it would have remained as a confusion to me forever.

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