Python capitalize the first letter only

Understanding capitalization rules and exceptions

While capitalizing the first letter of a string may seem like a simple task, specific string structures and cases may lead to unexpected results. Built-in Python methods like .capitalize() and .title() come with certain limitations and behaviors that everyone isn't aware of.

Pitfalls with .title() & .capitalize()

The .title() method capitalizes every first letter, but it also lowers all subsequent letters. This isn't ideal when strings contain proper nouns or acronyms. On the other hand, .capitalize() will not act on any letter following a digit at the start of a string.

Inspecting strings with leading digits

A string commencing with a digit introduces a unique challenge. Here, a custom function can get the job done:

import re

def capitalize_first_letter(s):
    return re.sub('([a-zA-Z])', lambda x: x.groups()[0].upper(), s, 1)

# Because '1st' isn't the same as '1St'!
input_string = "1st example"
result = capitalize_first_letter(input_string)
print(result)  # '1St example'
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In this snippet, re.sub() smartly takes the first alphabetic char and transforms it into an uppercase version. The rest of the characters remain original.

Capitalization preserving original cases

To preserve original cases in strings when capitalizing the first character, a loop is the friendliest solution:

def capitalize_first_letter(s):
    for i, char in enumerate(s):
        if char.isalpha():
            return s[:i] + char.upper() + s[i+1:]
    return s

# And we have a winner!
input_string = "hello world"
result = capitalize_first_letter(input_string)
print(result)  # 'Hello world'
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This is a simple loop technique that iterates and capitalizes the first alpha char. Don't forget to add error handling for strings bereft of any alphabets.

Wrangling complex string scenarios

In cases with complex string formats (mixed cases, spaces, non-alphabetic characters), you might need to craft a custom solution. Neither .capitalize() nor .title() are equipped to handle such tasks singlehandedly.

A keen eye with string.capwords()

Need to capitalize the first letter of every word in a sentence or a phrase? Welcome aboard, string.capwords():

import string

input_phrase = "hello world"
result = string.capwords(input_phrase)
print(result)  # 'Hello World'
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Every word in the phrase gets its moment in the sun with capitalized first letters. Ideal for titles and headings but beware of this tactic for token-wise capitalization in a string.

Careful precision with regex

If precision is what you're seeking, regex won't disappoint! The ([a-zA-Z]) pattern is an armory to target the first alphabet specifically:

import re

# When '123abc' goes undercover and comes out as '123Abc'!
input_string="123abc"
result = re.sub('([a-zA-Z])', lambda x: x.groups()[0].upper(), input_string, 1)
print(result)  # '123Abc'
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The lambda expression within re.sub() ensures the first alphabet gets capitalized. All in all, you enjoy the best of both worlds: capitalize after leading symbols/digits and keep the rest of the string intact.